REPORT DOCUMENTATION PAGE BEFORECOMPETIGORM1. Rr AT R. . GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

/7 ~5. CX RPRT 46 P6400-COVCREZSPhotochemistry of Metal-Metal Bonded Transition ntri TcialRe t

Element Complexes.,

It CIM&T R GRA NT NUMVIER(@)[rak . Wihon,,James L./Graff,,.Jh CLuong,N, J00014-75-C-088P

CarCarol L./Reichel_..jmd John L./Robbins

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PVMGPAM ELEMENT, PROJECT. TASKDepatmen of hemi~ryAREA %ý WORK UNIT NUMBERS

Massachusetts Institute of Technology NR01/7

Cambridge, MA 02139.RPTAKI I. CONTROLLING OFFICE NAME AND ADDRESS W RTOS

Office of Naval Research Qeb 1 1ýDepartment of the Navy -13 NUcMBER OFPAGESArlington, Virginia 22217 34UEOPAE

14, MONITORING AGENCY NAME a- ADDRESS(It d~ifferent from Conltrollind Office) 15. SECURITY CLASS. (of this repo~rt)

UNCLASSIFIEDfIsa DECL ASSI FICATION/ DOWNGRADING

SCHEDULE

IS. DISTRIBUTION STATEMENT (of this Report)

Approved for public release; reproduction is permitted for any purposeof the United States Government; distribution unlimited.

17. DISTRIBUT.'ON STATEMENT (of the abstract en~tered In Block 20, it different from Rhport)

Distribution of this document is unlimited.

14. SUPPLEMENTARY NOTES

Prepared and accepted for publication in the American Chemical DTSociety Symposium Series, 1981, Malcolm Chisholm, ed. _EC

19. KEY WORDS (Canntiet on rver.,. side It nec...avy anct Identify by block number)

photochemistry, metal-metal bonds, transition elements -A-

C) 4 ABSTRACT (Contlnuo anl reverse side If necwessay and identify by bled. number)

">ýTransition metal complexes that have direct metal-metal bonds have been theLA..3 objects of intense -interest from the point of view of geometric and electronic

structure, synthesis anid catalysis. Low-lying electronic excited states ofmetal- netal bonded complexes often involve significant changes -in the electrondensity associated with the metal-metal bond, zompared to the ground electronicstate. Accordingly, study of the photochemistry of metal-metal bonded complexnot only provides potential new reaction chemi~stry but also provideinghinto, and confirmation of, the electronic structure. This symposium volumeI

DD I *lq 1473 EDITION OF I NONI OZ kvr

UNCLASSIFIED

ý.L(UVMTY CLASSIFICATION OF THIS PAGE(Wlhn Date Itnere• .

affords, ds an opportunity to record the state of the field of metal-metal bondohotochemistry. The aim of this article is to summarize recent reseerchresults from this laboratory and to place them in perspective in relationto results from other laboratories

t *K

•I~n I

LINCLASS IfiT ED _______

SECU4I1TY CLASSIFICATION OF THIS PAOG(WmII. Dale RJ•lieO

NIL

OFFICE OF NAVAL RESEARCH

CONTRACT NO0014-75-C-0880

Task No. NR 051-579

TECHNICAL REPORT NO. 25

PHOTOCHEMISTRY OF METAL-METAL BONDED TRANSITION ELEMENT COMPLEXES

by

Mark S. Wrighton, James L. Graff, John C. Luong, Carol L. Reichel,

and John L. Robbins

Department of ChemistryMassachusetts Institute of Technology

Cambridge, Massachusetts 02139

Prepared for publication in the American Chemical Society Symposium Series

December 29, 1980

Reproduction in whole or in part is permitted for anypurpose of the United States Government A o 'ct' t' I

This document has been approved for public release and ' 'sale; its distribution is unlimited. 17,

Di:- t. Ii t I /

Avolil:J, ; Codc's

!

Photochemistry of Metal-Metal Bonded Transition Element Complexes

Mark S. Wrighton, James L. Graff, John C. Luong, Carol L. Reichel,

ýand John L. Robbins

}¶.assachusetts Institute of TechnologyCambridge, Massac-husetts 02139

(Prepare.A foi publication in the ACS Symposium Series, "Reactivityof MetalrMetal Bonds", M. H. Chisholm, ed.)

IA c*Addre~s correspondence to this author, ;

r[

I . -

- 1~

*Mark S. Wrighton, James L. Graff, John C, Luong, Carol L. Reichel,and John L. RobbinsDepartment of Chemistry, Massachusetts Institute of Technology,Cambridge, MA 02139 U.S.A.

Transition metal complexes that have direct metal-metal bondshave been the objects of intense interest from the point of view ofgeometric and electronic structure, synthesis and catalysis (1., 2,3, 4). Low-lying electronic excited otates of metal-metal bondedcomplexes often involve significant changes in the electrondensity associated with the metal-metal bond, compared to theground electronicstate. Accordingly, study of the photochemistryof metal-metal bonded complexes not only provides potential newreaction chemistry but also provides insight into, and confirmationof, the electronic structure. This symposium volume affords usan opportunity to record the state of the field of metal-metal bondphotochemistry. The aim of this article is to summarize recentresearch results from this laboratory and to place them inperspective in relation to results from other laboratories.

Complexei studied in this laboratory have included low-valent, organometallic clusters having two, three, or fourmetals. A priori the photoexcit(-d complexes can be expected toundergo any reaction that is possible, but the rate constant fora given process will be different for each electronic state.Whether reaction occurs with measurable yield from a given excitedstate depends on the rate of the reaction relative te internalconversion of the excited state to a lower lying excited state.From an examination of products alone, therefore, it is notalways possible to quantitatively assess the relative reactivityof the varlnus electronic states. But when photochemical productsdiffer £rom tvose obtained by thermal activation alone, profoundeffects from the redistribution of electron density can beinferred. In some cases the excited state may simply give thesame product as from thermal activation of the ground state.However, when photoreaction occurs it should be realized that theconversion to product occurs within the lifetime o thee'cited state.Even the longest-Lived excited metal complexes are of the orderof10- s (5, 6) in lifetime and the longest-lived metal-metal bondedcomplex in 298 K fluid solution is of tl.e order of _10-6 a inlifetime (7). Thus, excited state reactions of any kind must

-~ -.

2

have large rate constants compared to those for the thermallyinert ground state species. Measuring the rate constant for agiven chemical reaction in two different electronic states isa procedure for directly assessing the relative chemicalreactivity of the two states.

In the sections below we will describe in detail the knownphotochemistry of di-, tri-, and tetranuclear metal clusters.Results will be discussed in simple electronic structural terms.

Photochemistry of Dinuclear Complexes

a. Complexes Having Single M-M londs and Tf-d or a aLowest Excited States. It is apparent from the description ofthe electronic structure of Mn 2 (CO)10, (8, 9), Scheme I, thatcomplexes having a single M-M bond could have low-lying excitedstates that should be more labile than the ground state withrespect to eit...r M-ligand di.-sociation or M-M bond cleavage.In particulat, Mn2 (CO) 1 0 and its third row analogue, Re 2 (CO) i,exhibit low-lying excited states arising from rr-d - a* andob a a transitions (8, 9, 10, 11). These high symmetry

(OC) 5 M (Oc) 5 M-M(CO) 5 • M(CO) 5

d 2 2 ---------------------------------- d22x-y --- x-y

d - - 2 d 2dz2 4. • da

t -H- - - .- - . I

Scneme I. One-electron level diagram for I2-

Mn2 (CO)10, Re2 ( C0)O)10, W2(CO) 10 etc.

- )

3

"d 7 -d 7 1, honodinuclear dimers were the first to be subjectedto a detailed photockemical invpstigation (10, 11). Irradiationresulting in IT-d - Y or ab -• a transitions results in clean,quantum efficient, homolytic scission of the M-M bond to yieldreactive, 17-'ralence electron radicals, equation (1) (10, 11).

MCh' - 2M(CO) (i) z

M 2 (CO) 10 alkane -5

The generation of 17-valence elv-tron radicals is consistentwith flash irradiation of the heterodinuclear dimer MnRe(CO)iothat yields radical coupling p ducti according to equation (2)I. hv C' 22MnRe(CO) h%0 alk _,+ Mn2(CO), 0 + Re2(CO) lO(2)

(1, 12). Photogeneration of MnRe(CO)'jo from irradiation of both

Mn2(CO,'10 and Re2(CO)10 also occurs,and such cross-coupling hasbecome a definitive test of whether homolytic metal-meta' bondcleavage occurs. Kinetics of recovery of ground state nn2(CO)10absorption after flash irradiations are consistent with formationof Mn(CO) 5 radicals from the excitation (13).

Irradiation of M2 (C0)10 in the presence of halogen donorsprovides chemical evidence consistent with the quantum efficientgeneration of 17-valence electron radicals (6). Chemistryaccording to equation (3) occurs with a disappearance quantum

313 nm (ob - U*).Re 2 (CO) 10 CCI 4 2Re(CO) 5 CI (3)

yield of 0.6 (10), showing that greater than half of the excitedstates yield the cleavage reaction. The efficiency of homolyticcleavage may be greater since cage escape of Re(CO)5 radicalsmay be less thin unity. There is a solvent viscosityeffect on the disappearance quantum yield of M2 (CO) 1 0 in thepresence of 12,consistent with a solvent cage effect (11).

In polar solvents (pyridine, THF, alcohols, etc.) thephotochemistry of simple M-M bonded systems seeis to be differentbased on the products observed (14, 15). For example, irradiationof Mn2 (CO)10 can give Mn(CO)5-. But this chemistry very likelyoriginates from the 17-valence radical as the primary product.Disproportionation of the 17-valence electron species, perhapsafter substitution at the radical stage, can account for theapparent heterolytic cleavage. If the excited state reactionis truly dissociative, as the evidence from cross-coupling inalkane suggests, there should be little or no influence fromsolvent.*

Population of the o orbital in M2 (CO) 1 0 should labilize theM-CO bond, (8, 9) and such is likely the case. However, promptCO loss apparently does not compete with scission of the M-Mbond, Irradiation of M2 (CO)1 0 in the presence of potentialentering ligands such as PPh3 does lead to substitution (5, 6, 11),

but the principal primary photoproduct suggests a mechanism otherthan dissociative loss of CO to give M2 (CO),. For example,

-%•now . . .. .. .

.4

irradiation of Mn2 (CO) 1 0 in the presence of PPh 3 leads to mainly14n2(CO),(PPh 3 )2 rather than the expected Mn2 (CO) 9 PPh 3 (11).This led to the postulated mechanism represented by equations(4)-(6). The substitution iability of 17-valence electron radicals

hv~

S 2(CO) 10 2 .Mn(CO) (4)

2 "Mn(CO) 5 + 2PPh 3 - 2 ,Mn(CO) 4 (PPh 3 ) (5)

2 "Mn(CO)(PPh -i (CO) 3) 2 (6)

has been elegantly elaborated by T, L. Brown and his co-workers(L6, 17, 18, 19). ,

The population of the G orbital in M2(CO) 1 0 obviouslygives rise to considerable lability of the M-M bond. The groundstate of these molecules is inext; the high quantum efficiencyfor photoreaction means that virtually every excited stateproduced yields radicals. The excited state cleavage rates canbe concludod to be >1010 s-, since no emission has ever beendetected from these species and the reactions cannot be quenched1.1-by energy transfer. The excited state cleavage rate of >,101 s-is many orders of magnitude ?.arger than from the ground stateand reflects the possible consequence of a one-electron excitation.Te question of whether both depopulation of Qb and population ofa are necessary is seemingly answered by noting that iriadiationat the low energy tail of \*he Tr-d ÷ a absorption gives a quantumyield nearly the same as that for qb - * excitation. However,the relative quantum yields do not reflect the relative excitedstate rate constants for reactIon. It is only safe to concludethat either rr-d - a or ob + a results in dramatic labilizationcompared to ground state reactivity and that the rate constantis large enough in each case to compete with internal conversionto an unreactive state. Another ambiguity is that the stateach'.eved by a given wavelength may not be the state from whichrceaction occurs. The reactive P'.ates, for example, could betriplet states that only give :ise to weak absorptions from theS~ground state.-Electrochemistry aad redox chemistry provide possible ways

of assessing the consequence of one-electron depopulation orpopulation of orbitals. The M2 (CO•o spgcies suffer rapid M-Mcleavage upon reduction (population of a ) or oxidation(depopulation of GOb) (20. 21). But cyclic voltammetry onlyreveals that the cleavage rate from the radical anion,M2(CO) 1 0, or radical cation, MK(CO)'ot, is >-101 s'-. This iscertvi.nly consistent buL direct measurements of the rate remainto be done,

Comparison of the photochemistry and the thermal reactivity Iof M2(CO) 1 0 is of interest. Considerable effort has been expended

to show that thermolysis can lead to M-M bond cleavage. The"activation energy from M-M dissociation has been examined andcorrelated with the position of the 0 b - o* absorption band from

I e -.

:7V

a number of derivatives of Mn2(CO)>.o The question is: what isthe relative importance of M-ligand cleavage v.,, M-M cleavage inthe ground state? A recent report bears op. this question,Thermal reaction of MnRe(CO)lo with PPh.J was examined (22).The interesting finding is that the primary products do notinclude Mn2 (CO) 8 (PPh 3 ) 2 which would be expected if Mn(CO) 5 wereproduced in the rate limiting step (see equations (4)-(6)). Thus,the ground state pathway for this complex appears to be mainly COloss, not Mn-he bond cleavage. The photoreactivity appears to bedominated by Mn-Re bond cleavage (11). A difference in M-ligandvs. M-M cleavage is seemingly established. The excited stateresults in a relatively low barrier to M-M cleavage compared toXM-ligand cleavage. Since the excited state reaction is so cleanand quantum efficient (vide infra) it is tempting to generalizethe specificity found for excited state M-M' cleavage inMnRe(CO) 1 0 . But this generalization would require that the groundstate have a lower activation energy for M-ligand cleavage thanfor N-M cleavage. Such is apparently not the case for certainspecies such as (f-CG5 1 5) 2 Cr 2 (CO) 6 that likely exist in solutionin equilibrium with the (r-CsH5)Cr(CO)3 radical (23). The species

[(Us-allyl)Fe(CO) 3] 2 also exists in the ,onomeric form insolution (24). Weak M-M single bonds du exist and the groundstate M-M cleavage can clearly dominate the chemistry. Light mayserve only to accelerate the observed rate.

There are now known a large number of other photosensitivedinuclear complexes that can be formulated as having a 2-electronsigma bond (25-29). Designation of the electronic configurationby the dn configuration of the 17-valence electron fragment thatwould be obtained by homolytic cleavage allows a convenientcataloguing of the speies studied so far, The transitionmetal-metal bonded complexes in the summary, Table I, have allbeen shown to undergo efficient M-M bong hemoly sis upon Photo-excitatlon (Li, 25-29). Examples of d -d d,-d7, d9 -d',d5 -d', d7 -dY, an-d d--d9 are all known. All of the complexesexhibit an optical absorption spectral feature in the near- v

or high energy visible that can bý attributed to the ob atransition. Generally, a T-d -* 0 feature is also observable"at lower energy than the Ob + o*, and irradiation at this

energy results in homolysis but with a somewhat lower quantumyield. None of the complexes listed are complicated by bridging

ligands (see succeeding section) and none give significantyields for CO loss as the primary photoprocess.

All of the M-M single bonded complexes can be formulatedwithin the framework suggested by Scheme II. The 17-valenceelectron radicale studied all havc about the same group electro-negativity based on the ability to predict thk vosition of the

b + T* absorption of heterodinuclear dimers, M-M' from the

Jb -0 (Y energy iti M-M and M'-M' (28). Significant ioniccontribution to the M-M' bond will likely result in a situationwhere M-M' bond t leavage is heterolytic or where some otherreaction. M-ligand cleavage for example, dominates the excited

__,1

6

I a-d N

ca-d

- -0

•:'•-d 1T•-d

Scheme II. General one-electron level. diagrari for M-M'single bonded complexes.

state processes. Irradiation of the (q-C 5 H5 )M"CO) 3X (M - Mo,W) (30), (n-CsH 5)Fe(CO) 2 X (31), or Re(CO) 5 X (32) (X - halide orpseudohalide) yields CO extrusion; in these cases the 0b "b" *transition is very high in energy.

Complexes that exhibit a formal two-electron sigma bondbetween a transition metal and main group metal ar,-.of Interestbut have received relatively little study. Species such as(CH 3 ) 3SnMn(CO) 5 are thermally quite rugged but are photo-sensitive (33). Irradiation of (n-CsHs)Mo(CO) 3 SnMe 3 in thepresence of P(OPh) 3 yields CO subatitution (34). The questionof course is whether the primary photoreaction is loss of CO orhomolysis of the Sn-M bond. The complexes R3SnCo(CO)nL4-nrepresent an interesting set of Sn-M bonded complexes, becausethe 17-valence electron Co-fragment is of d9 configuration andthe complex has only one low-lying c orbital. For example,Scheme III shows a simple, but very adequate, one-electron leveldiagram for Ph 3 SnCo(MO) 3 L. Irradiation of this complex doesnot lead to efficient formation of Co2 (CO) 6 L2 that would b-ian expected cross-coupling product from photogenerated radicals,Rather, CO substitution occurs with a quantum yield of -0.2 at*366 nm (35). The lowest excited state is very likely a T--d-oexcitation and does not appear to result in sufficient Sn-Mlability to compete with Co-CO cleavage within the excitedstate lifetime. The related system R3SiCo(CO) 4 likewise onlygives CO extrusion upon photoexcitation (36). In b-th of thesesystem& the 0 b -0 0 is likely much higher in energy than thent-d -, o transitions. Even if the 4- excitation isachieved it is not clear the Sn-M or Si-M bond cleavage cancompete with M-CO cleavage rates in the excited state. Itappears that bonds between group IV elements and transition

I metal carbonyls are not efficiently cleaved compared to theefficiency for M-CO cleavage.

L(OC) 3 Co o L(OC) 3 Co-Sn.Ph 3 SnPh 3 '

d l d 2 -2

...-- - -_.

d dxZ, yz

Scheme Ill. One-electron level diagram for a highsymmetry d9 , 17-valence el<•ctron radicalcoupled to Ph3Sn.

b. Photochemistry of Bridged, M-M Single Bonded Complexes. Theunsupported M-M single bonds, Table I, give clean rupture of theX-14 bond upon photoexcitation. Bridged M-M bonds represent asituation that could differ significantly since the bridginggroup could prevent the dissociation of the 17-valence electronfragments. The doubly CO-bridged Co2(CO) 8 , and (fl-CsHS) 2M2 (CO)4(M - Fe, Ru) species have been examined. Of these, the mostinteresting is (n-C5Hs)2Ru2(CO)4 which exists as a non-bridgedor bridged species, equation (7), in solution at 298 K. The

Ru- Ru ____R-R -. ,(7)

ratio of the two forms depends on the solvent with alkane solventsgiving approximately a 1/1 ratio, while polar solvents such asCH 3 CN give nearly 100% of the bridged form (37, 38). The 366 nmnquantum yield for reaction according to equation (8) is

"C 0.1 14• (C) 2 (N-C 5RH)Ru(CO) 2 Cl (8)(-5H5) 2Ru2(O 4 0.1i M CCI455

solvent

IAA

8

essentially the same in CH3CN and in isooctane where the fractionof incident light absorbed by the non-bridged form changesdramatically (38), The lack of a change in the quAntum yieldsuggests that the bridged species is cleaved as efficiently as thenon-bridged species by excitation corresponding to transitionsthat lead to population of the (1 orbital.

The (n-CIH5)zFe2(C0)4 is fully bridged in any solvent.Irradiation at 298 K in the presence of CC14 accelerates thethermal rate of formation of (n-C 5 H5)Fe(C0) 2 CI (38, 39). Light-induced cross-coupling of (fl-C 5H 5) 2 Fe 2 (C0) 4 with the radicalprecursors M2 (CO) 1 0 , (M - Mn, Re) (n-C 5 H5 ) 2 M2 (C0) 6 (M - Mo, W) isobserved (28, 40L), consistent with clean photogeneration of the(•-CsHs)Fe(C0) 2 radical. Irradiation of (n-C 5 H5 ) 2 Fe 2 (CO)4 at lowtemperature in solutions containing P(OR) 3 results in an inter-mediate hypothesized to be a CO-bridged species that has aruptured Fe-Fe bond (41). This intermediate can be used to accoulnt

I" for the reactions of photoexcited (n-C 5 15 ) 2 Fe 2 (CO) 4 withoutinvoking the prompt photogeneratLion of free radicals. However,the radical cross-coupling is likely best explained by theultimate generation of the free radicals but this may not berequired if the lifetime of the CO-bridged species not containingthe Fe-Fe bond is pufficiently long. In any event, population ofstates where the a orbital is populated leads to a significantlyweakened 14-M interaction and net cleavage does occur. PromptCO ejection does not seem to be an important process for either

the Fe or Ru species.The Co 2 (CO) 8 also exists as bridged and non-bridged forms in

solution (42). Irradiation of Co2 (C0) 8 and (n-C 5H5 ) 2M2 (CO) 6(M - Mo, W) leads to the expected radicitl coupling product

(i-CsH 5)M(CO) 3Co(CO) 4 (28). But the quantum efficiency is notknown. Irradiation of Co 2 (CO) 8 at low temperature has been shownto lead to loss of CO, equation (9), and the reaction is

C(CO) hv Co°2(08 low temperature 2 (C0) 7 + CO (9)

matrix

wavelength dependent but quantum yields have not been reported (43).Reaction according to equation (9) leads to the suggestion that

dissociative loss of CO can become the dominant photoreaction whenthe cage effect becomes severe enough to preclude separation

of the 17-valeace electron radicals. The importance of the

bridging CO's In 298 K solutions is difficult to assess at thistime. Some of the usual chemiLal probes are unsatisfactory here

because the Co 2 (CO) 8 is labile thermally in the presence ofpotential entering ligands. The Co(CO)4 radical does not seem to

react rapidly enough with CC14 to provide a reasonable measure ofthe efficiency of the photogeneration of Co(CO)4 (27, 28). Thesituation is further complicated by the fact that the Co(CO) 4 X(X - Cl, Br, I) species are thermally labile, even if formed.

"I..

Intuitively, it would seem that bridging ligands wouldresult in significantly lower quantum yields for M-M cleavape.Such is not found experimentally. If a retarding effet is tobe quantitated it will likely come from a direct measurement ofthe excited state rate constants. Table II summarizes thekey results Lo date.

c, Photocheisistry of Dinuclear Complexes Having Multiple M-MBonds. The cornerstone example of strong, multiple M-M bondsis the Re I Re quadruple bonded Re 2 C18

2 - (44). This substancewas found to suffer Re • Re bond cleLvage from an upper excitedstate produced by optical excitation ia CH3CN solution,equation (10),(45). The lowest excited state, associated with the

Rei2- hv (0Re2 Cl8 CH3 C 2ReCl 4 (CH3 CN) 2 - (0)

'5 + transition (46), is unreactive and apparently does notdisrupt the bond enough to yield cleavage. Even the upper excitedstate suffers cleavage via attack of the CH3 CN as determinedfrom flash photolysis studies (47). The photoinduced cleavageplaces an upper limit on the Re z Re bond energy, but theenergetics are obscured by the fact that reaction is not dissoci-ative homolytic cleavage. The Pe2 CI8 2- and Re 2 Br82' were foundto be luminescent from the +64 excited state (48, 49). In the

L first report (48) describing the low temperature (1.3 K) emissionof Re 2 X8

2 • (X CI, Br) and Mo2 CI8 - the emission oi-100ns lifetime was attributed to the triplet state, butsubsequently (49) it was reported that the complexes Re 2 X8

2 -

andMMoaClz(PR3) 4 are emissive in fluid solution at 298 K withlifetimes in the 50-140 ns regime. The detailed study (49) ledto the crk-lusion that the emission originates from a distorted singletstate. 2elated MoMo quadruple bonded complexes apparently do emit fioma triplet state at low terperature (4ý with a lifetime of -2 us forMoa(OZCCF)4 at 1.3 K..'Te MCI ýIPi species have been found to undergophotooxiddtion in chlorocarbon soludna (4,5_, bit like the ýhotocheridstryfor RMX 2 - (45, 4L, the reaction occurs from an upper excited state.

TheMo=Mo quadruple bonded species K4Mo2 (SOI,) 4 has beeairradiated in aqueous sulfuric acid solution (50). The photo-chemistry proceeds according to equation (11). The eisappearance

H4 +N+,2 (SO ) 4 - ½H2 + Mo 2(SO4 ) 4 (11)(~aq) 2 4-- 2 2

quantum yield at 254 nm was found to be 0.17, but the reaction canalso be effected with visible light. This system exemplifies thenotion that if the lifetime of the excited state is not controlledby dissociative M-M bond cleavage, then bimolecular processes maybe possible. The Mo2 (S0 4 ) 4 4- is another example of a multipleM-1 bond that cannot !e efficiently cleaved in a dissociativemanner. Similarly, photoexcitation of MoZX04- (X - CI, Br) in

aqueous acid results in photooxidation. Clean formation ofMo2 Cl8 H3- can be observed. Again, rupture of the M & M quadruplebond is not found (50).

S• _ '+•.... • .... • : • ,Y .. ......... ... + ,.• ... • I•coo•

10

Racent studies of the hydrocarbon soluble complexesrn-CSRN ) 2 Cr 2 (CO) 4 (R - H, Me) and (rj-CS 11 ) (ij-CsMe5 )Cr 2 (CO) 4that are formulated as having a Cr Cr triple bond, (51, 52)show that the dominant photoreaction is loss of CO accordingto equation (12) (53). The tricarbonyl ,-pecies accounts for the

(ýJ-C5R5)2Cr2(CO) 11 R, hr (12)5R 5 2 2( 4 a-k- ('I-C 5 R 5 ) 2 Cr 2 (CO) 3 +CO (12)

incorporation of 13CO upon irradiation o1 (u1 -CsR) 2 Cr 2 (CO) 4under a 13a) atmosphere. Irradiation of(11-C 5 116 ) (q-C5R5 )Cr 2 ((O) 4 produces no detcctable amount of(n-CsHs) 2 Cr2 (C0)4 or (')-C.e4 •),,C,,(CU) 4 and no measurable(1-C51•5 )(t-C 5 Mes)Cr2 (CO) 4 is termed when a 1/1 mixture of the

symmetrical species is irradiated. The negative results from thtnattempted cross-coupling experiments rult out an important )olefor the prompt generation of 15-valence ,lectron fragments fromphotoexcitatioti of the Cr I Cr triple bonded complexes. Thequantum yield for loss of CO, equation (12), is wavelengthdependent; irradiation at the lowest absorption (,600 nm) result:iin no reaction, but near-uv firadiation gives a quantum yield of>10-2 (53), The wavelength dept,adence is reminiscent of C02(C0)',In low temperature matrices where the potential fragments aretethered by the bridging groups and the cage effects of thematrix (43).

Irradiation of the (0-C 5 1l 5 ) 2 V2 (CO) 5 , VIV double bondedspecies results in loss of CO, not V=tV bond cloavage upon photo-excitation (54). The crucial result comes from an experimentwhere the (1-'CsU1)2V2(CO)-, is irradiated at -50 0 C in the presenceof PEt 2 Ph in T-F solution. The only product observed is(Ti-C 5•li ) 2 V2 (CO) 4 PEt 2 Ph.

From the studies of the quadruple, triple, and double bondedcomplexes examined thus far, it will prove difficult to photo-chemically cleave multiple metal-metal bonds in a dissociativefashion. Photochemical cleavage of multiple bonds is not taboo,since the light induced cleavage of 02 and C02 is well known,But in metal complexes it appears that other processes such assolvent attack on the excited state, electron transfer, and ligand"dissociation can lead to excited state deactivation bforo bondrupture can occur. As seen in the sunmiary in Table 1l1, dissoci-ative clavage of a multiple metal-metal bond remaills to beaccomplished, Also, upper excited state reaction is the rule inthe multiple bonded systems.

d. Photochemistry of Dinuclear M-M Bont!ed Complexes hayinjChaseyrantfer Lowest Excited St-tes. Complexes such aoRC(CO) s~(C&O3(l ,10-i )I i e troline (5) anldPhsSnRe(CO) 3(I,10-1 -henmAthroline) (7, 5_) and related cauoiix.•cin u1kaformulated as having a M-H or W'-N single bond. Table IVPsummarizes the known photoprocessos for such complexes. Thelowest excited state in th• complvxes hts been identified asarising from a (N-M)ob -4 it phen charge t ransfer transition(7, 55, 56). Importantly, the orbital of tenninatlon of the

-,NN

transition is not the o* orbital associated with the M-14 or M-11'sigma bond. The orbital of termination is localized on thecharge acceptor ligaud and population of it is not expected toseriously disrupt the M-14 or M-M' bonding. Consistent with thisassertion is the fact that the complexes are reversibly reducibleon the cyclic voltammetry time scale (56). However, the oxidationof the complexes is not reversible and cyclic voltammetry showsthat M-M or M-M' cleavage occurs for the radical monocation ata rate of >10 s-' (56).

The lowest energy electronic transition of the Re 2 (CO) 8 -(1,10-phenanthroline) and related complexes is expected to labilizethe M-M bond in the sense that the M2 -core is "oxidized" by theintramolecular shift in electron density. Excitation has beenshown to yield M-M bond cleavage; reaction according toequation (13) is representative (55). The quantum yield of -0.2

Re 2 (CO) 8 (l,10-phenanthroline) y CIRe(CO) + CIRe(CO)Cci4 5

(l,10-phenanthroline) (13)

was found to be independent of the excitation wavelength from550 unm to 313 Pm, This wavelength independent quantum yield isconsistent with reaction that originates from the (M-M)ab ÷+ -n

phen CT state that is found to yield emission (but no photo-reaction) at low temperature. This result shows that depopulationof an M-M core bond level labilizes the M-M bond sufficientlyto allow cleavage within the lifetime of the excited state, The77 K emission lifetime was found to be extraordinarily long(-95 psec), but emission was not detectable at all at 298 K wherephotoreaction occurs with a quantum yield of -0.20. It ispossible that emission is not observable at 298 K because M-Mbond cleavage occurs too fast; this would indicate that the M-Mdissociation rate in the excited state significantly exceeds105 s-1. Given that the ground state dissociation is slow(<10-6 s-1) for these thermally inert (298 K) systems, even anexcited state rate of 105 s- reflects an increase in dissociationrate of >lQ11 compared to the ground state. An excited state rateof >10' s-- is consistent with the lower limit of 103 s-1cleavage rate of the radical monocation generated in electro- ,chemical experiments.

Study of Ph3SnRe(CO) 3 (l,lO-phenanthroline) resulted in thefirst direct detei-minrvtion of the rate constant for excitedstate cleavage of the M'-M bond (7, 56). The key is that thiscomplex is emissive from the reactive state under the conditionswhere the cleavage reaction also occurs. Measurement of theemission lifetime (1.8 x 10-6 s) and the photoreaction quantumyield (-0.23) give a rate constant of 1.3 x 105 3-1 at 298 K for

J!ij

12

the dissociation represented by equation (14). The wavelength

5'hL-pl0_hnanthroline , Ph 3 Sn + ReCCO) 3 L (14)

! 2CH2C12/0,5 M CCI 4

independence of photoreaction quantum yields (488-313 nm) and theability to equally efficiently quench the lifetime, emission,and photoreaction with anthracene confirm that the emissivestate is also the reactive state. The -10 s- excited statecleavage rate is consistent with the lower limit of 10 3 sfrom cyclic voltammetry for cleavage of the radical monocation.

The relatively long lifetime of the lowest excited state ofPh3 $nRe(CO)3 (1, lO-phenanthroline) allows fast bimolecularprocesses to compete with the cleavage of the M'-M bond (7).For example, anthracene, having a triplet energy of -42 kcal/mol,quenches the excited state (-50 kcal/mol) at an essentiallydiffusion controlled rate by electronic energy transfer. Theexcited state can also be quenched by electron transfer,equations (15) and (16) (56). Both of these processes are

k 15 +

[Ph 3 SrnRe(CO) 3 LL + TMPD - TMPD" + [Ph 3 SnRe(CO) 3 L3 (15)

, k2 + +

[Ph3 SnRe(CO) 3L] + MV - MV" + [Ph 3SnRe(CO) 3L]' (16)

"21PD=N,N , ,N -tI.tame thfi/-phenyf/nedLf/ M•-LdimA-4,4"h~iddnium

significantly downhi.ll and the rate constants k.ý and k 1 6 arethose exected for diffusion controlled reactions. The reductionto form h 3 SnRe(CO) 3 L]- results in no net chemical change, sincethe back electron transfer is fast and the electron added to theRe complex in equation (15) is localized on L. The excited stateoxidation though results in net chemistry, since chemistryaccording to equation (17) competes with the back electron

[Ph 3 SnRe(CO) 3 LQ' + , lvnt 7 Ph 3Sn" + SRe(CO) 3 L+ (17)

transfer. The unimolecular rate constant k17 is >I03 s"! from theelectrochemistry and from the photochemistry (equation (14)) thevalue of k 1 7 could be greater than 105 a-'. The one-electronoxidants lead to production of 1.8-electron SRe(CO) 3 L+ products; +

this leads to the conclusion that the cleavage of [PhaERe(CO) 3L]"yields Ph 3E" and Re(CO) 3 L+, not Ph 3 Sn + and .Re(CO)jL.

The experiments with the variouý -M-Re(CO) 3L speciesestablishes that population of the o level is not required toachieve sufficient M-M bond lability to yield homolytic cleavagewithin the lifetime of the excited state. However, when the sigmabond order is reduced from one to approximately one-half by thedepopulation of the ab level1 the rate constant for M-M bondcleavage appears to be only -l10 s-1. By way of contrast, M-Mbond cleavage seems to occur with a rate of >10'° a for species

L. ... . . .- i

13

such as Re 2 (CO) 1 0 where the sigma bond order is reduced from oneto zero by the (b + I* excitation.

Photochemistry of Trinuclear Complexes

Table V summarizes the key photochemistry of trinuclearmetal-netal bonded complexes. The first noteworth; photochemicalstudy of trinuclear complexes concerns Ru 3 (CO)1 2 . This specieswas found to undergo declusterification to mononuclear fragmentswhen irradiatud in the presence of entering ligands such as CO,Pph•oxr, ethylene. The intriguing finding is that thermal reactionof Ru 3 (CO) 1 2 with PPh 3 results in the substitution productindicated in equation (18) whereas irradiation yields t.-e mono-nuclear species given by equation (19) (57). These results

Ru 3 (CO) 1 2 P-Ah Ru3(CO) 9 (PPh 3 ) C18)

Ru (CO) h Ru(CO) 4 PPh 3 + Ru(CO) 3 (PPh 3 ) 2 (19)R3(O 12 PPh-34 -3 3

suggest that Ru-Ru bond cleavage occurs in the excited statewhereas Ru-CO dissociation occurs in the ground state. Anelectronic structural study shows that the orbital of terminationfor the lowest energy excited states of Ru 3 (CO) 1 2 is a * with

respect to the Ru 3-core (58). The disappearance quantum yieldsfor Ru 3 (CO) 1 2 , Fe 3 (CO)1 2 and Ru 3 (CO) 9 (PPh 3 ) 3 are all in the rangeof 10-2 for entering groups such as CO or PPh 3 and are independentof entering group concentration (59, 60). 'rhle ctroksctructureis consistent with primary photoreaction as represented by

vr equation (20), and the overall low quantum yields are consistent

(20)

with efficient closurc to regenerate the metal-metal bond.Irradiation of Ru3(CO)7.z (57, 60), Fe3(CO)12 (60), or

Ru3(CO)9(PPh3)3 (59) under CO cleanly leads to mononuclearcomplexes. This fact seems to rule out dissociative loss of COas the dominant reaction from the excited state. If loss of CO

were the dominant process, the presence of CO would simplyretard the decomposition of the cluster. The lack of aneffect from high concentrations of entering group (1-pentene orPPh3) on the quantum yield for photodeclusterification ofFe3(CO)12 or Ru3(CO)12 (60) is consistent with this conclusion.

The lowest excited states of triangular trinuclear complexes,like dinuclear complexes, can also be labile with respect to loss

a _ _ __b......

.2

14

of CO as well as metal-metal bond cleavage, Irradiation ofOs 3 (CO) 1 2 unier conditions where Ru 3 (CO)1 2 is declusterifiedleads to substitution of CO, equation (2L) (60, 61). This

Os3 (CO) 1 2 Ph3 + Os 3 (CO) (PPh 3 )1 (21)3 12 PP1h3 n 33-3

n w lli 10, 9

change in photoreactixity may be due to the fact Lhat the lowestexiited state isO -*o in the Ru3(CO) ia casc whereas it isO* -o* in the case of 0s 3 (CO)j 2 . An alternative explanation maybe simply that the stronger Os-Oý- bouds have lower dissociationrates while Os-CO and Ru-CO cleavage rates are similar, In anyevent, there is a striking difference in the qualitative featuresof M3 (CO) 12 (M = Fe, Ru) vs. Os 3 (CO) 12. Absolute photoreactionquantum yields for any phoLoreactLion are small, but it does appearthat M-M boud cleavage dominates for M = Ru or Fe whilc M-COcleavage dominates for N - Os.

4 The clean photodeclusterification represented by equation (22)

2 H3 Re3 (CO)1. 2 3--+ 3 U2 Re2 (CO) 8 (22)

is a case where photoexcitatLion leading (L either Re-Re cleavageor dissociative loss of CO could allow rationalization of the Aobserved chemistry (62). The observed quantum yield of -.0.1 isthe highest observed for a triangular metal-metal bonded system.This high quantum yield and the fact that: the Re-Re bonds aicbridged by 11 aLouud; suggest that the dissociative loss of CO isthe likely result oC p1hoLoexcitaLion in this case.

Photolysis of the complex RCCo0(CO),Y (R - 1, CH 3) under anH 2 atmosphere has been reported to yield declusterification withlow quantum efficiency (63). As in the case above it is not clearwhether reaction begins with M-M or M-CO bond cleavage; theexcited state should be more labile than the ground state withrespect to either process.

In contrast to the generally low quantum yields for thetriangular-M3 systems, 0s 3(CO) 12 C12 , that has only two Os-Osbonds, undergoes photoinduc, M-HNI bond cleavage with a highquantum yield (61). This result lends support to the assertionthat the tethered diradical center, equation (20), may be importantin giving net quantum yields that are low compared to species suchas Mn2 (CO)IO, A possible contributLion to low quantum yields forthe triangular-M3 core systems is the fact that the ono-electronexcitations promoted by optical absorption are not simplylocalized labilizatioi of two of the three bonds.

Photochemistry of Tetrnnuuclear ComiplexesComplexes having the tetrah,'drane-M,, core have six direct

M-M bonds and it isý not likely that ooe-(,lectron excitation will

result in enough bonding disruption to e-:trude mononuclearfragments. If an M-M bond dou, cleave, -quatLion (23) , cleavage of

I.

1./

(23)

two other M-M bonds would be required in order to generate themononuclear fragment. Cleavage in the sense suggesLed byequation (23) can seemingly occur, though, since photoreactiortaccording to equation (24) has been observed (63). If

Co4 (CO) 1I) ---- 2 Co2 (CO) 8 (24)

dissociative loss of (CO occurs the presence of added CO wouldseemingly only lead to back reaction with no net chemical change.Likewise,11FeCo(CO)12 yields Co2(CO)B when Irradiated under CO (63).

The quantum yields' for these reactions are low and it is notclear where tOe inefficiency lies: is the M-M bond cleavage(equation (23)) a low quantum yield process or is the reformationof the M-M so fast thit it is inefficiently trapped by CO?

Irradiation of ir4(CO) 17 in the presence of (MeCO 2 ) 2 C2results in the retention of an Ir 4 complex but the Ir4-core in theIr4(CO)M(MeCO2 )2 C,}1 product is a rectangle. Such a product couldarise from either CO loss or from trapping of a photogenerateddiradicicl. As ir, the trinuclear M3 (CO) 1 2 complexes, the resul/for the Ir4 species (1r4 retention) compared to the Co. species(fragmentation (63)) may sigual a trend in M-M bond retentionfor the third row systems where M-M bonds are expected to bestronger.

Not surprisingly, the tetranuclear Fe 4 (CO) 4 (n-C 5 ' 4 )4 , thathas a tetrahedrane-Fe,, core with the CO's triply face bridging,is photoinert in solution with respect to Fe-Fe or Fe-CO bondrupture (65). In the presence of halocarbons such as CCl 4 thereis a clean photooxidation reaction, equation (25), resulting from

FeCO -) H CTTS [Fe4 )4(C) -CH 5 )4 ]+ (25)e 4 (O 4 (- 5 5 4 -" CCl 4 [ 4 4 55)

irradiation corresponding to absorption due to a charge transferto solvent transitioti. Ferrocene exhibits a similar photo-reactivity (66) ; to t.xLvnd the comparison it is noteworthy thatthe potential for [fe ricenium/fe•.rocene and [Fe 4 (CO) 4 (T)-C 5 H5 )]+/0is nearly the same (Q7) , the CTTS is at about the same energyand intensity for a ,iven halocarbon, and the quantum yields forphotooxidation (after correction for intramolecular absorption)are quite similar (6W) . The resilience of the Fe4 (CO' 4 (-CSI1 5 )4with respect to light induced bond cleavage reactions allows the

4

16

otservation of the photooxidation process. A similar situationappears to exist for the quadruple bonded Mo2 (S0 4) 4

2 andMo 2Cl 4 (PR 3) 4 described above (50).

Quantum inefficient ligand dissociation does appear to bethe primary chemical result from photoexcitation ofP4?.-4(CO) 12 (68). Irradiation at 436 or 366 nm in the presenceof an eatering ligand proceeds according to equation (26). The

HRu(CO) u (CO) (26)4 4( 12 L.-P(OMe) 3, PPh 4R4 11

reaction has the same quantumi efficlency (5 + I x 10-3) forL - P(OMe) 3 or PPh 3 and for a variation in concentration of Lfrom 0.01 to 0.1 M. These observations ,upport the promptgeneration of 114Ru 4 (CO)i1 from photoexcitation. The complexundergoes substitution Lhermnally but phowoexcitation accelcikatesthe rate dramatically. Photoexcitation t f 114Os 4 (CO) 12 does givechemistry and pho oreactlon may begin w lilh dissociative lossof CO subsequent to photoexcitation (69).

Irradiation of H4 Re4(CO)12 does not result in any significantphotoreaction (70). The lack of any Re-l<e bond cleavage may beassociated with the fact that the Re-Re bonds have multiple bondcharacter (71). The H4 Re.(CO)12 exhibiti" emission from the lowest

excitc-1 state. This finding promp-eO a comparl-nn of the excitedstate decay of the D4 Re 4 (CO) 12. Generally, the highest energyvibrational modes are important in non-radatLive dcecay (72), andfor metal complexes the highest energy M-1, vibrations may be mostimportant. The hydrogon atoms in H4Re4k('O) . are believed to betr.pl~y face bridging (71) with a Re 3-11 st retching frequency of-1023 cm-1 (73). The H-Re4(CO) 12 comple> emits in hydrocarbon

solution or as the pure solid at 298 or 77 K. ThV, emission(-14,300 cm-1) onset overlaps the absorption and thus a largedistortion of tlhe comple;, upon excitation do.es not appear tooccur. The emis,!1oi, can be quenched by tle triplet quencheranthracene, having a triplet energy of -42 kcal/mol (74), Thelifetime is In the range 0.1 - lb tjsec depending on conditionsand is of the order of 20-30% longer for the lil subst itutedcomplex. Likewise the emission quantum yields for theD Re 4 (CO) 12 are 20-30% longer than for the lH species. lhitis,replacing IlI by 211 in IllRe 4 (C0) 12 has the expected effect ofreducing the rate of non-radiatLive decay. But the effect doesnot lead to a situation where the excited decay Is dominated bythe radiative dec..y rate.

The studies of the tetrahiedranc-ML clusters, Table VI, showquite generally that the imporlance ot dissociative processeswithin the excited statc ifetinie is low. Clean, but low quantumyield, photoreactions are detect!blve In some cases. The resistancecf the complexes to declusterificat ion may be exploited to studynnd utilize bimoleoular processes, atid in the 'ase of the'I 4Re4(C0) 12 the reactions and non chemical , non-radiatlve decay

are sufficlently low that the excited stIte lifetime allows

17

rp,,iative decay to be observed. Such long-lived excited statesmay allow the development of a bimolecular excited state chemistry

of such species.

Acknowledgements. W( thank the National Science Foundation and

the Office of Naval Resear,-h for partial support of the research

described herein. Support from GTE Laboratories, Inc., is alsogratefully acknowledged. M.S.W. acknowledges support as aDreyfus Teache -Schol;ar grant recipient, 1975-1980.

Literature Cited

1. Cotton, F.A, Ac_. Chem. Res., 1969, 2, 240.2. Cotton, V.A. Ace. Chem. Res., 1978, 11, 225.3. Cotton, P.A. and Wilkinson, G. "Advanced Inorganic

Chemistry" 4th ed., John Wiley and Sons: New York, 1980,Chi. 26, pp. 1080-1112, and references cited on p. 1112.

4. Trogler, W.C.; Gray, H.B. Acc. Chem. Res., 1978, 11, 232.5. Balzani, V.; Carissiti, V. "Photochemistry of Coordination

Compounds", Academic Press: New York, 1970.0 6. Geoffroy, G.L.; Wrighton, M.S, "Organometallic Photo-

chemistry", Academic Press: New York, 1979.7. Luong, J.C.; Faltynek, R.A.; Wrighton, M.S. J. Am. Chem.

Soc., 1979, 101, 1597.8. Levenson, R.A.; Gray, H.B. J. Am. Chem. Soc., 1975, 97,

6042.9. Levenson, R.A.; (Gray, H.B.; Caesar, G.P. J. Am. Chem. Soc.,

1970, 92, 3653.10. Wrighton, M.; Br.desen, D. J. Organometal. Chem., 1973,5.,C35.11. Wrighton, M.§.; Ginley, D.S. J. Am. Chem. Soc., 1975,

97, 2065.12. Evans, G.O.; Shetine, R.K. J. Inorg. Nucl. Chem., 1968,

30, 2862.13. Hughey, IV, J.L.; Anderson, C.P,; Meyer, T.J.

J. Organometal. Chem., 1977, 125, C49.14. Allen, D.M.; Cox, A.; Kemp, T.J.; Sultann, Q.; Pitts, R.B.

"J.C.S. Dalton, 1976, 1189.15. Burketc, A.R.; Meyer, T.J.; Whitten, D.G.

J. Organomet. Chem., 1974, 67, 67.16. Byers, B.H.; Brown, T.L. J. Am. Chem. Soc., 1975, 97,

947 and 3260; 1977, 99, 2527.17. Absi-Halabi, M.; Brown, T.L. J. Am. Chem. Soc., 1977,

99, 2982.18. Kidd, D.R.; Browi, T.L. J. Am. Chem. Soc., 1978, 100, 4103.19. Brown, T.L. Ann. New York Acad. Sci., 1980, 333, 80.20, Lemoine, P.; Giraudeau, A.; Gross, M. Electrochim. Acta,

1976, 21, 1.21. Lemoine, P.; Gross, M. J. Organomet. Chem., 1977, 133, 193.22. Scnnenberger, D.; Atwood, J.D. J. Am. Chum. Soc., 1980,

L02, 3484,

22. Adams, R.D.; Collins, D.E,; Cotton, F.A J3. Am. Chem. Soc.,

1.974, 96, 749,24. Murdoch-, 11.D.; Lucken, E.A,C. lHelv.. Chim. Acta, 1964,

47, 1517.25. Wrighton, M. S.; Cinley, 1J.S. J. Am. Chem. Soc., 1975,

97, 4246.26. Ginley, D.S.; Wrighton, M,S, J. Am. CThem. Soc., 1975,

97, 4908.27. Abrahamson, H.B. ; Wrighton, M.S. J3. Am. Chem. Soc.-, 1977,

99, 5510.28. Abrahamison, It. B. ; Wrighton, M. S. Inorg. Chem. , 1978, 17,

1003.29. Reichel., C .L.; Wrighý,:. KS. J. Atl. Pheni. Soc.., 1979,

10,6769.30. Alway, D.C. ; Barnett, '1 W. Inr. ,ý 1980, 19-, 1533.31. Alway, D,.C.; barnett, l(,W. Chr~hem. , 1978, 27, 2826,32. Wrighton, M.S. ; Morse, 1).L. ; Gray, P.B-. 1 0 t t Cson, D.K.

J. Am. Chem. Soc., 1976. 98, 1111.33. Clark, ti.c. ; Tsai, J.H. lnr.Chem.., 1966, -5, 1407,34. King, R.B.; ?annel1, K,11. lnorg . Cie. , 1968, 7, 2356.35. Revichel, C.L.; Wrighton, M~.S. to bc sUbilitted.36. Rei,:hel, C. L. ; W righton, M.S. T-norý-_ Chemn., 1980, 19, 0000.37. F"ischer, RD,; Vogler, A.; Noack, K. J. Organomnet. Chem.,

1967, 7, 135.38. Abrahamson, 11.1. ; PalazzoL~to, M.C. ; Reichel, G.E.;

Wrighton, M.S. J. Aml. Chem. Soc., 1979, 101, 41.23.39. Giannotti, C.; Merle, C. Jrgon. Che i. , 1976, 105, 97,40. Ginley, D.S. Ph.D.' Thesis, MiT,1976.41, Tyler, D. R.;. Schmidt , M.A.; Gray, 11D. B.J. Am. Chem. Soc. ,

1979, 101, 2753.42, Noack, A. 11elv. Chim. Acta, 1964, .,7, 1.055 and 1064.43. Sweany, R.L.; Brown, TLb. Iiuorp. Cem. , 1977, 16, 421.44. Cotton, F. A.: Harris, C,11. - lnoI&!jhc1m. , 1965, 4, 330.45. Geo ff roy, GA.L.; G ray, H. B.;- 1auuiwo ld, G. S. J. Am. Chein. Soc.,

1974, 96, 5565.

46. Trogler, W.G.; Cowukin, C.D.; Gray, R.B.; Cotton, F.A.J3. Am. Chem. Soc. , 1977, 99, 2993.

47. Flemming, R.H. ; Geof iroy, GL. ; Gray , l1.B. ; Gupta, A.;

Hammond, G.S.; Kliger, D.; Mislkowski, V.M. J3. Am. Chem.-Soc., 1976, 98, 148.

48. iTroýgler, W.G-.; Solomon, E.I. ; Cray, H.B. ; Inorg, Chem.,1977, 16, 303"t and Trogler, W.G.; Solomon, E.I.; Trajberg,I.; Bali1hausen, C.J.; Gray, H.B. lporg. Cem., 1977, 16,828.

49. Miskowski, V.M.;, Coldbeck, R.A.; K~lger, D.S.; Cray, H.B,Lno . Chem., 1197, 18, 86.

50. Erwin, D.K.; Geoffrov, G.L.; Cray, 1I.B.; Hammond, G.S.;Solomon, E.1. ; Trogler, W.C. ; Zagars, A.A. J, Atl. Chiem.Soc., 1977, 99, 3620; Troeler. W.G. ; Gray, R.B. Nouv. J.

Chim, , 1977, 1, 475 and Trogler, W.C, ; Erwin, DKGdeoiffroy, G.L.; Gray, 1-LB. J. Am. Chem. 3oc, , 1978, 100,1160. -

19

51. King, R.E.; Efraty, A. J. Am. Chem. Soc., 1971, 93, 4950.52. Hackett, P.; O'Neill, P.S.; Manning, A.R. J.C.S. Dalton,

1974, 1625.53. Robbins, J.L.; Wrighton, M.S. submitted for publication.54. Lewis, L.N.; Cauluon, K.G. Inorg. Chem., 1980, 19, 1840.55. Morse, D.L.; Wrighton, M.S. J. Am. Chem. Soc., 1976,

98, 3931.

56. Luong, J.J.; Faltvnek, R.A.; Wrighton, M.S. J. Am? Chem.Soc., 1980, 102, 0000.

57. Johnson, B.F.G.; Lewis, J.; Twigg, M.V. J. Organomet. Chem.,1974, 67, C75 and J.C.S. Dalton, 1976, 1876.

58. Tyler, D.R.; Levenson, R.A.; Gray, H.B. J. Am. Chem. Soc.,1978, 100, 7888.

59. Graff, J.L.; Sannor, R.D.; Wrighton, M.S. J. Am. Chem.Soc., 1979, 101, 273.

60. Austin, R.G.; Paouessa, R.S.; Giordano, P.J.; Wrighton, M.S.Adv. Chem. Ser., 1978, 168, 189.

61. Tyler, D.R.; Altobelli, M.; Gray, L.B. J. Am. Chem, Soc.,1978, 102, 3022.

62. Epstein, R.A.; Gaffney, T.R.; Geoffroy, G.L.; Gladfelter,W.L.; Henderson, R.S. J. Am. Chem. Soc., 1979, 101, 3847.

63. Geoffroy, G.L.; Epstein, R.A. Inorg. Chem., 1977, 16, 2795and Adv. Chem. Ser. , 1978, 168, 132.

64. Heveldt, P.F.; JIohsuon, B.F.G.; Lewis, J.; Raithby, P.R.;Sheldrick, G.M. .T.C.S. Chem. Comm., 1978, 340.

65. Bock, C.R.; Wrighton, M.S. log. Chem., 1977, 16, 1309.66. Brand, J.C.D.; Snedden, W. Trans. Faraday Soc., 1957,

53, 894.67. Ferguson, J.A.; Meyer, T.J. J. Am. Chem. Soc., 1972,

94, 3409.68. Graff, JoL. ; Wrighton, M.S. J. Am. Chem. Soc., 1980, 102,

2123.69. Bhaduri, S.; Johnson, B.F.G.; Kelland, J.W.; Lewis, J.;

Raithby, P.R.; Rehani, S.; I.heldrick, G.M.; Wong, K.;McPartlin, M. J.C.S. Dalton, 1979, 562.

70. Graff, J.L. ; Wrijhton, M.S. to be submitted.71. Saillant, R.B.; Barcelo, C.; Kaesz, H.D. J. Am. Chem. Soc.,

1970, .92, 5739.72. Turro, N.J. "Modern Molecular Photochemistry", Benjamin

Cummings: Menlo Park, California, 1978.73. Lewis, J.; Johnson, B.F.G. Gaz. Chim. Ital., 1979, 109,

271.74. Calvert, J.G.; Pitts, Jr., J.N, "Photochemistry",

John Wiley & Sons: New York, 1966.

20

w~ 001 0 U j4 4 7

'4-1 14 t.CO

- 00 U) 00 0~- .0

44 4J ) 4

o co 4J .-4 0W 0 A0 0)

0,4r r40 4-14- 0 -H-r

P4I

0

0)r u--

.- o oý o

Cd -4 %-' 04.' ) r. ( -4-1 Q l L

Q0 q -4 04 w coq cu- - - 0c ) 4Q

000 L ( ) - u Huu0 -1u 4c) u0

wO 41 0) w Q u I u~ I -

04 o 0 0 4 m (n u 0 ~0 0 V UP4 0 0 Q) 0-4 0 ~ 04 10 0 0 0~ IV

p 0 ý0 0 p 0 pw -H ~0 k 0 p

o p. 0 f. o u co0 0m

O w.. il-4 I1-4 4"-4 Q4-4 ) 4-4 Q)4-4 144

04 U

U 0')

r-*0 ~CON

4-4 .-.

o ,"%0r L. C) C)

-r-- Cl L

u 0 ýr o

po 1-1 - 0 ' .

C)~~C 0'''

ON 0i w w'~ ('

('4 P L P

*~e -4)- V))'.-I LP4 ,-4.. L)

coo

)ý4 ýl )~

o4) r-4d

0 -

.Y-4

,-4 00

0 U)0o 4 C . 4

0-H

'-4

~'W' -04 U) cE

p. 144 04- 0 0::3

W 0 V0

0 0)

0o i 0 V _

u w ) C- p4 4

(- 4 4 0 4

lid-u

0

22

4-40 '

S0

00 cl 0

0 ILI I

*V 4JC Q A4 ýL4 ý4 C30

.r ( JC c-r4J 0 0 ) :j 10 -~ #0 0.0 -4 4 W 4J 14~4 "3~4 -r40

w~ 40 0 to s-40ý 0c p,- -r k~ J-4

4J~ V) r_ -4 ) ýQ) a4j' Q) ri-

0 )0)HO4.10 Q4.) w& ) "o.&r WJ r4 i W44 414f 10 m-

4 :

I-A m-

0 00

CN 0

If Z ) O I

(3)U 44-~ ~

5.4 u- 0 0'T 0

00 + e.- l +04 2 0 - I.

-r - 01 0- %(ý 0ý N00 C-4 0 0) -r

0) 0 No 0 N) N- N10 P-4 P4 IX4 0'.. P4.

H C4 ~(U N M Nwt~ L-fl W e L( 0 0

'4-4 0r Un ý: 'ýL

0 C1N43~ ý1: 0

0)0 C*410 00

(3)co

8- -0) U(

23

0~4.)0 r4

0

44 44)

0

0 0) 0 *i41

0.) -4r r 0J 0

CL 0

#.a~ ~ 000) cn~ W

C') "_4 4~~~ e-.1r H H CnI 0d

10 P4 UU1 0co0 WOi ;4 0 -,

P4 fC) 41 4-1 N 0-

(n) 0 n

0 4.)

'Xi *H 0 -

0 0

1.4

4) ~ Ln : 0 C) C

0 -.:r -

pq

4.)0- + ý

4-j 1 r-.

4cJ 0

0) 0u-'

PL4 00 1- C

441 u zo) >

P4-

Ln H0.i . o -4H 4 Vl

H- 00 In

0 0C14

~~2E-4 E-4~

I~.7

24

ILTable IV. Primary Excited State Processs of

SMRe(CO) 3 (l,l0-phenanthrolin,!) (7, 55, 56)

* Complex Pho .oprocess

Re(CO)sRe(CO) 3 (l,lO-phenanthroline) Emit;sion at 77 K

Re-Ile Bond Dissociation ati98 K

Ph3E~e(CO)3(l,10-pheiianthroline) Emis;sion at 298 or 77 K

(E - Sn, Ge) E-Re Bond Dissociation

Electronic Energy Transfer

Electron Transfer

Table V. Photochemistry of Trinuclear M-M Bonded Complexes.

Complex Primary Photoreaction (Ref.)

M 3 (CO)12 (M Fe, Ru) M-M Cleavage is likely dominant(57,59,6_0) (low overall quantum

yield).

Ru3 CO) 9 (PPh 3) 3 Ru-Ru Bond Cleavage (59) (lowoverall quantum yield).

L Os3 (CO) 12 Dissociative loss of CO (60,61)0 O(low quantum yield),

Os3(CO)1 2C12 Os-Os Bond Cleavage; Linear

Structure; High Quantum Yields (61)

1it43 (CO) (M = Mn, Re) CO loss or M-M Bond Cleavage (62)

RCCo 3 (CO) 9 (R - CH3 , H) CO loss Co-Co Bond Cleavage (6•3

i~I:

i_

Table VI. Photochemi!ýtry of Tetranuclear Complexea,

Complex Photoprocess (Ref.)

Co4 (CO)12 Likely Co-Co Bond Cleavage; Co2 (CO) 8

formed under CO (63)

{FeCo2(CO)2 Primary process likely M-M Bond Cleavage (63)

Fe 4 (CO) 4 (n-C 5 H5 ) 4 Inert in Hydrocarbon; Photooxidation viaCTTS in Halocarbon (65)

Ir 4 (C0) 1 2 Derivative with Ir 4 unit retained canbe formed (64)

H14 Ru 4 (CO) 1 2 Loss of CO; Low Quantum yield (68)

H 4OS4(CO)I2 loss of CO (69)

H 4Re 4(CO)2 Photoinert; Emits at 298 or 77 K (70)

!AI AI

•'

-.- -t --I

SP472-3/AI 472:GAN:716:ddc78u472-608

TECHNICAL REPORT DISTRIBUTION LIST, GEN

No. No.Copies

Office of Naval Research U.S. Army Research OfficeAttn: Code 472 Attn: CRD-AA-IP800 North Quincy Street P.O. Box 1211Arlington, Virginia 22217 2 Research Triangle Park, N.C. 27709 1

ONR Branch Office Naval Ocean Systems CenterAttn: Dr. George Sandoz Attn: Mr. Joe McCartney536 S. Clark Street San Diego, California 92152 1Chicago, Illinois 60605 1

Naval Weapons Centera Attn: Dr. A. B. Amster,

A Chemistry DivisionChina Lake, Californ~a 93555 1

No~k ew Yorký 100 1Naval Civil Engineering Laboratory

ONR Western Regional Office Attn: Dr. R. W. Drisko1030 East Green Street Port Hueneme, California 93401 1Pasadena, California 91106 1

Department of Physics & ChemistryONR Eastern/Central Regional Office Naval Postgraduate SchoolAttn: Dr. L. 11. Peebles Monterey, California 93940 1Building 114, Section D666 Summer Street Dr. A. L. SlafkoskyBoston, Massachusetts 02210 1 Scientific Advisor

Commandant of the Marine Corps

Director, Naval Research Laboratory (Code RD-I)Attn: Code 6100 Washington, D.C. 20380 1Washington, D.C. 20390 1

Office of Naval Research

The Assistant Secretary Attn: Dr. Richard S. Iiillerof the Navy (RE&S) 800 N. Quincy Street

Department of the Navy Arlington, Virginia 22217 1Room 4E736, PentagonWashington, D.C. 20350 1 Naval Ship Research and Development

CenterCommander, Naval Air Systems Command Attn: Dr. G. Bosmajian, AppliedAttn: Code 310C (H. Rosenwasser) Chemistry DivisionDepartment of the Navy Annapolis, Maryland 21401Washington, D.C. 203601

n Naval Ocean Systems Center

Defense Technical Information Center Attn: Dr. S. Yamamoto, MarineBuilding 5, Cameron Station Sciences DivisionAlexandria, Virginia 22314 12 San Diego, California 91232 1

Dr. Fred Saalfeld Mr. John BoyleChemistry Division, Code 6100 Materials BranchNaval Research Laboratory Naval Ship Engineering CenterWashington, D.C. 20375 1 Philadelphia, Pennsylvania 19112 1

S• 7 -3 /3472:GAN: 716 :ddcSP472-3/A3

78u472-608

TECHNICAL REPORT DISTRIBUTION LIST, GEN

No.

Dr. Rudolph J. MarcusOffice of Naval Research

Scientific Liaison Group

American EmbassyAPO San Francisco 96503

Mr. James Kelley

DTNSRDC Code 2803Annapolis, Maryland 21402

2

KL .

_

SP472-3/A 11 472:GAN :716 :ddc7 8u 4 7 2- 6 0 8

TECHNICAL REPORT DISTRIBUTION LIST, 359

No. No.Copies Cospes

Dr. Paul Delahay Dr. P. J. HendraDepartment of Chemistry Department of ChemistryNew York .'niversitv University of SouthhamptonNew York, New York 10003 Southhamtton S09 5NHI

United KingdomDr. E. YeagerDepartment of Chemistry Dr. Sam PeroneCase Western Reserve 11niversitv Department of ChemistryCleveland, Ohio 41106 1 Purdue University

West Lafayette, Indiana 47907Dr. D. N. BeniionDepartment of Chemical Engineering Dr. Royce W. MurrayBrigham Young University Department of ChemistryProvo, Utah 84602 1 University of North Carolina

Chapel Hill, North Carolina 27514Dr. R. A. MarcusDepartment of Chemistry Naval Ocean Systems CenterCalifornia Institute of Technology Attn: Technical LibraryPasadena, California 91125 1 San Diego, California 92152

Dr. J. J. Auborn Dr. C. E. MuellerBell Laboratories The El-ctrochemistrv BranchMurray Hill, New Jersey 07974 1 laterials Division, Research

& Technology DepartmentDr. Adam Hieller Naval Surface Weapons CenterBell Laboratories White Oak LaboratoryMurray Hill, New Jersey 07974 Silver Spring, Maryland 20910 1

Dr. T. Katan Dr. C. Good•manLockheed Missiles & Space Globe-Union Incorporated

Co, Inc. 5757 North Qreen Bay AvenueP.O. Box 504 Milwaukee, Wisconsin 53201Sunnyvale, California 94083

Dr. J. Boechler

Dr. Joseph Singer, Co-'e 302-1 Electrochimica CorporationNASA-Lewis Attention: Technical Library21000 Brool-pnrk Road 2485 Charleston RoadCleveland, Ohio 44135 1 Mountain View, California 94040

Dr. B. Brummer Dr. P. P. Scbmridt

EIC Incorporat.d Department of Chenistrv55 Chapel Street Oakland UniversityNewton, Massachusetts 02158 1 Rochester, Michigan 48063

Library Dr. F. RichtolP. R. Mallory and Company, Inc. Chemistry DepartmentNortl jest Industrial Park Revsselaer Polytechnic InstituteBurlington, Massachusetts 01803 1 Troy, New York 12181 1

II

SP472-3/B13 472:GAN: 16 :dic

78u4 72-608

TECHNICAL REPORT DISTRIBUTION LIST, 359

No. No.

Coo ies C-o6pi !3

Dr. A. B. Ellis Dr. R. P. Van Duyne

Chemistry Department Department of Chemistry

University of Wisccnsin Northwestern University

Madison, Wisconsin 53706 Evanston, Illinois 60201

MH Wrighton. Dr. B. Stanley Pons

SChe v Tla;Ntment Department of Chemistry

Massoc <e titute University of Alberta

! echnology Edmonton, Alberta

.mbridge, Massachusetts 02139 1 CANADA T6G 2G2

Larry E. Plew Dr. Michael J. Weaver

Naval Weaoons Support Center Department of Chemistry

Qode 30736, Building 2906 Michigan State University

Crane, Indiana 47522 1 East Lansing, Michigan 48824

S. Ruby Dr. R. David Rauh

DOE (STUR) EIC Corporation

600 E Street 55 Chaoel Street

Washington, D.C. 20545 1 Newton, Massachusetts 02158

Dr. Aaron Wold Dr. J. David Margerurm

Brown University Research Laboratories Division

"Department of Chemistry Hughes Aircraft Company

Providence, Rhode Island 02192 1 3011 Malibu Canyon RoadMalibu, California 90265

Dr. R. C. ChudacekMcGraw-Edison Company Dr. Martin Fleischmann

Edison Battery Division Department of Chemistry

Post Office box 28 University of Southampton

Bloomfield, New Jersey 07003 1 Southnapton 509 5NH England

Dr. A. J. Bard Dr. Janet Osteryoun'

University of Texas Department of Chemistry

Department of Chemistry Statt. University of New

SAustin, Texas 78712 1 York at BuffaloBuffalo, New York 14214

Dr. M. M. NicholsonElectronics Research Center Dr. R. A. Osteryoung

Rockwell International Departme'nt of Chemistrv

3370 Miraloma Avenue State University of New

Anaheim, California I York at BuffaloBuffalo, New York 14214

Dr. Donald W. ErnstNaval Surface Weapons Center Mr. James R. Moden

code R-33 Naval Underwater System.q

Whitt Oak Laboratory Center

Silver Soring, Maryland 20Q10 1 Code 3632Newport, Rhode Island 02840

__2i

S- .,.

SP472-3/AI5 472:GAD:716:ddc78u472-608

TECHNICAL REPORT DISTRIBUTION LIST, 359

No. No.CopiesCopies

Dr. R. Nowak Dr. John KincaidNaval Research Laboratory Department of the NavyCode 6130 Stategic Systems Project OfficeWashington, D.C. 20375 1 Room 901

Washington, DC 20376i i Dr. John F. Hioulihan

Shenango Volley Campus H. L. R•obertson,,•Penns•ylvania State University Manager, ElectrochemicalSharon, Penncylvania 16146 1 Power Sonices Division

Naval Weapons Support CenterDr. X. G. Sceat.' Crane, Indiana 47522Department of ChemisLryUniversity of Rochester Dr, Elton CairnsRochester, New York 14627 Energy & Environment Division

Lawrence Berkeley LaboratoryDr. D. F. Shrivor University of CaliforniaDepartmet of Chemistry Berkeley, California 94720,lorthwesteru UniversityEvanston, Illinois 60201 Dr. Bernard Spielvogel

U.S. Army Research OfficeDr. D. H. 1hitmore P.O. Box 12211Department of Materialt Science Research Triangle Park, NC 27709Northwestern Univers ityEvanston, Illinois 60201 Dr. Denton Elliotts

Air Force Office ofDr. Alan Bevick Scientific ResearchDepartment of Chemistry Bldg. 104The University Boiling AFBSouthampton, S09 5N1) England Washington, DC 20332

Dr. A. Him',NAVSEA-5433NC #42541 Jefferson Davis HighwayArlington, Virginia 20362 1

3

SP482-3/A23 472:GAN:716:ddc78u472-608

TECHNICAL REPORT DISTRIBUTION IIST, O51A

No., No.

Copies Copies

Dr. M. A. El-Sayed Dr. M. RauhutDepartment of Chemistry Chemical Research DivTisionUniversity of California, American Cyanamid Company

Los Angeles Bound Brook, New Jersey 088051Los Angeles, California 90024 1

Dr. J. I. ZinkDr. E. R. Bernstein Departuent of ChemistryDepartment of Chemistry University of California,Colorado State University Los AngelesFort Collins, Colorado 80521 1 Los Angeles, California 90024

Dr. C. A. Heller Dr. D. HaarerNaval Weapons Center IBMCode 6059 San Jose Research CenterChina Lake, California 93555 1 5600 Cottle Road

San Jose, California 95143Dr. J. R. MacDonaldChemistry Division Dr. Jol-n CouperNaval Research Laboratory Code 6130Code 6110 Naval Research LaboratoryWashington, D.C. 20375 1 Washington, D.C. 20375

Dr. 0. B. Schuster Dr. William M. JacksonChemistry Department Departcent of ChemistryUniversity of Illinois Howard UniversityUrbana, Illinois 61801 1 Washin~ton, DC 20059

Dr. A. Adamson Dr. George E. WalraffenDepartment of Chemistry Departrent of ChemistryUniversity of Southern Howard University

California Washiniton, DC 20059Los Angeles, Califurnia 90007

11Ms S.nWragshton oDep-t ofM~assachus <nstitute of

ii ece