organologam lengkap
TRANSCRIPT
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Outl ine
1. Review on coordination chemistry2. The 18-electron rule
3. Limitations of 18-electron rule
4. Oxidation number
5. Coordination number and geometry
6. Effect of complexation
7. Differences between metals
Chapter 1. General properties of organometallic
complexes
1. The Organometallic Chemistry of the Transition Metals, Robert H. Crabtree,
3rd Edition, 2001, Chapter 1-2.
2. Organotransition Metal Chemistry, Akio Yamamoto, 1986. Chapter 1-4.
3. Organometallic Chemistry, G. O. Spessard, G. L. Miessler, Prentice-Hall:
New Jersey, 1997, Chapter 13.
References
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1. Review on coordination chemistry
Complexesor coordination compoundsare compounds composed
of a metal and ligandswhich donate electrons to the metal, e.g.
H3N: Co :NH3
H3N:
:NH3H3N Co NH3
H3N
NH3
NH3
NH3
..
..
3+
or
3+NH3
NH3
Ligands: a molecule or ion that has at least one electron pair that can be
donated.
The electron pair can be: lone pair, p-bonding electron pair, or
s-bonding electron pair.
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M M M
H H
M
H SiR 3
M
lone pairs, e.g.
pbonding e- pairs, e.g.
sbonding e- pairs, e.g.
H3N: H3N: M H2O: H2O: M.. ..
Classi f ic at ion of l igands
A). Based on nature of the donating electron pairs, ligands may be
classified as Lone pair donor, p-bonding electron pair donors, s-bonding
electron pair donors.
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1) Lone pair donors
M L
NH2-CH2-CH2-NH2(en).. ..
R-O-..
..: L : :-
In terms of bonding:
..
L
..
L
filledempty
Lor
H3N: :PR3:CO
M
Cl
M M
Note:
Ligands that donate
electron pairs to form M-L
sbond are also called s
donors.
MO description of Metal-Ligand interactions
L ligand
M
M L ligand
M L ligand
Bonding
Antibonding
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e.g. OR-
R-O..
.. :- M+(a)
R-O..
.. M
R-O
-:
M+(b)
MR-O
M
M M
filled
M
empty
orOR- OR- OR
-
OR-
(a)
Note: although ligands
such as OR-can form
pbond with a metal,
we usually don't
indicate suchinteraction in writing
the structures.
Ligands that donate electrons to metal to form pbond are called donor
How manyp
bonds can an OR form with a transition metal ion?
M ORM OR
M OR
Some lone pair donor ligands may have orbitals to form
M-L pbonds. ===> pdonor, pacceptor.
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:C O: C O: C O:
C O: C O:
+(a)
+
(b)
empty
M M
M
M
accept e-to form pbond (dpppback bonding)
CO is not only a sdonor, but also a pacceptor.
How many pbonds can a CO form with a transition metal ion?
Ligands that accepting electrons from metal to form p bond are calledacceptor.
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:C O: C O: C O:
C O: C O:
+(a)
+
empty
M M
M M
C O:+
empty
M C O:M
C OM C OM C OM
Can CO function as ap
donor?
TWO
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Types of lone pair donor ligands
strongpacceptor
weakpbonding
strongpdonor
lone pairdonor
CO, PF3 NH3, H-
CH3-
Cl-, OR-
s
p
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MoOC
OC CO
CH3
2.38 1.99 a)
b) IR, (CO)
:C O
2149 cm-1
C OH3B
2178 cm-1
OC Cr
CO
CO
CO
COOC
2000 cm-1
Evidences for dp
-pp
interactions.
* Take M-CO complexes as an example forp
accepter
Explanation?:C O :C OM C OM
C OM C OM
C OM C OM
-+
1s
1p
3s
2p
4s
2s
C O
2p
3s
1p
CC 2s
C O
C O
(anti-bonding)
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*Take M-OR complexes as an example forp
donor.
Mo Mo
O O
O
O
OO OO t-But-But-But-Bu
120o O
Nb
O
Tp
Tp
Tp = trypticyl
180o
In H2O, O atom has sp3
hybridization, leading to the A-O-B anglebeing about 104.5. However, in the above two complexes the angles
are 120and 180, respectively. Why ?
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2). p-bonding electron pair donors
C O
M
H2C CH2
M
HC CH
M M M
How can these ligands interact with M ?
Take CH2=CH2as an example.
M
1. e - from (C2H4) --->s(M)
M
2. e - from d(M) --->p
(C2H4)
HH
H H
C
C
p
C
C
p
C
C
C
C
HH
H H
M
Is CH2=CH2a pdonor or a pacceptor?
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Hapticity of ligands:
A ligand may have more than one way to bond to a metal center, e.g.
M M
or
M
CHCH2
CH2
H
H
HH
H
(Cp)
H
H
HH
H
M H
H
HH
H
M
M
In describing the number of atoms (n) attached to a metal, a short
hand hnis used. e.g.
M
(h1-C3H5)M
MH
H M
CH2
CH2
(h-H2)M (h-C2H4)M
MAg+ M
(h-C6H6)M (h-C6H6)Ag
+ (h-C6H6)M
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O
CH3
Fe(CO)3
Ru
Fe(CO)3
MeO +
Exercise. Give the hapticity of ligands in following complex
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3).s
-bonding electron pair donors
Relatively fewer stable complexes are known.
Typical examples:
MH
Hh-H2 OC W
OC
COPR3
PR3
H
H
h-H-SiR3 MH
SiR3Mn
OCOC
H
R3Si
agostic C-H Me2P Ti
Me2P
CH2Cl
Cl
MH
C Cl
CH2Hor M
H
C
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How can these ligands interact with M? Take H2as an example.
M
H
H
H
H
M
s-bonding p-bonding
Further Notes:
* Relative basicity of electron pairs:
lone pairs > pbonding electron pairs > sbonding electron pairs
* Therefore usual order of binding ability:
lone pair donor > pbonding electron pair donor > sbonding electron pair donor.
Consequence:
MH
HM
CH2
CH2
M PR3
CH2
CH2
CH2
CH
H
H
:PR3
Is H2a pdonor or a pacceptor?
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B) based on the nature of bonding interaction, ligands may be
classified as soft or hard.
Hard ligands: have low polarizability, especially those containing
period 2 donor atoms N, O, F, e.g. O2H, NH3, F-
Soft ligands: have high polarizability, they include:
a). Those with period three or subsequent donor atoms,
e.g.Cl, Br, S, P.
b). p-acceptors, e.g.CO, CS (carbon sulfide), H2, CN- (cyanide)
c). Those containing p-electrons, e.g.
C C C C
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Importance of the concepts of soft and hard ligands:
* Hard ligands tend to form stable complexes with hard metal ions,
(usually those at high oxidation state, e.g. Al3+, Fe3+, Cu2+, Ti4+, Pt(IV)).
* Soft ligands tend to form stable complexes with soft metal ions,
(usually those at low oxidation state, e.g. Mn(I), Co(I), Fe(II), Pt(II),
Pt(0) .......)
Examples:
AlF63-, Ti(OR)4are very stable complexes, but Pt(0)--F, Pt(0)--OR,
W(0)--F are very rare.
Cl Pt
Cl
Cl
-
stable
Pt(IV)
very rare
(Olefins are soft
base; Pt(II) is soft
but Pt(IV) is hard)
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Exercise. explain the following facts based on the concept of Soft-Hard
Acid-Base.
1) W(CO)6
is air stable, but W(NH3
)6
has never been observed.
2).
3). Low oxidation state complexes (most organometallic compounds) are
often air-sensitive, but are rarely water sensitive.
NH2
(NH3)5OsII
2+
- e- NH2
(NH3)5OsIII
3+
NH2(NH3)5OsIII
3+
slow
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Further notes on ligands
Chelation: Ligand attached through more than one atom usually separated
by one or more atoms. Chelating ligands are sometimes classified as beingbidentate(2 points of attachment), tridentate(three points of attachment), or
tetradentate(4 points of attachment).
Kappa convention (): The kappa convention is sometimes used toindicate the coordinating atoms of a polydentate ligand.
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2. The 18 electron rule
1)Thermodynamically stable transition metal organometallic
compounds are formed when the sum of the metal d electrons plus
the electrons supplied by the ligands equals 18.
In this way, the metal formally attains the electronic
configuration of the next noble gas.
Ni
CO
COOC
OCFe CO
OC
OC
CO
CO
Cr COOC
OC
CO
CO
CO
The 18 electron rule = 18 VE rule = inert gas rule = effective
atomic number rule (EAN rule). e.g.
Saturated complexes: 18 VE complexes
Unsaturated complexes:
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2) Ways to count valence electrons (VE)
# VE = valence e-of M (or Mn+) + e-from ligands
Two Models: covalent modeland ionic model
Covalent model: Both M and L are considered as neutral
# VE = valence e-of M + e-from ligands + charge
e.g.
Fe Cr
H
CO
COCO
OC
OC
2 2 x 5e
Fe 8e
18e
5 CO 5 x2e
H 1e
Cr 6e1- 1e
18e
C5H5
For TM, valence e-of M = group number
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For TM, valence e-of M = group number.
e.g. Fe, 8; Pt, 10
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Ionic model: Metal complexes are formed from Mn++ L + X-
# VE = valence e- of Mn++ e- from ligands
Fe Cr
H
CO
CO
CO
OCOC
2 2 x 6e
Fe2+ 6e
18e
5 CO 5 x 2e
H- 2e
Cr 6e
18e
C5H5-
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Rh
Cl
Rh
Cl
Rh
Cl
Rh
Cl
OCH3
Fe(CO)3 +
OCH3
Fe(CO)3
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Fe
OCOC
Mn
COOC
OC
CO
COW
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Rh Rh
OC
CO
(CO)3(PPh3)Fe Ir(CO)2(PPh3)
Ph2P
Co CoN
N
O
O
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Do the following complexes follow the 18e rule?
ReH7(PPh3)2
-
Cr
CO
COCO
H
OC
OCCr
CO
CO
CO
CO
OCRh
H
H
H
+
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R3P Ru
H
H
H
R3P
PR3
B H
H
R3P Ru
H
H
H
R3P
PR3
B NMe3
H
Additional exercise
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Additional exercise
Ir
N
COOC
Ir
OC CO
BPh3
Ir
OC CO
PPh3
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U
U: 7s25f36d1
2x8+6 = 22e
Lu Me
Lu: 6s24f146d1
2x5+1+17 = 28e
2).F-block metals do not follow 18e rule. e.g.
Why? because electrons can go to (n-2)f orbitals)
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3).Transition metal complexes ===> three classes
Class number of valence electrons 18e rule
I ....16, 17, 18, 19, 20 not obey
II ....16, 17, 18 not obey
III 18 obey
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Class II: 4d and 5d metals with weak-field ligands
Class II n(VE) 18n(d) n(VE)
ZrF62- 0 12
WCl6 0 12
WCl6
- 1 13
WCl62- 2 14
TcF62- 3 15
OsCl62- 4 16
PtF6 4 16
PtF6- 5 17
PtF62- 6 18
PtCl42- 8 16
Metals in relativelyHigh oxidation state
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Class III: Complexes with pgood acceptors
Class III n(VE) = 18
n(d) n(VE)
V(CO)6
- 6 18
CpMn(CO)3 7 18
Fe(CN)64- 6 18
Fe(CO)42- 10 18
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Explanation:
L M L
L
L
L
L
Take Oh as an example
MO levels in the absence of pacceptors.
* If is small, eg* can be
occupied.( < pairing energy)
===>
eg* (antibonding): 0 - 4e
t2g(nonbonding): 06e
a1g
, t1u
, eg(bonding): 12e
Total e-:
Minimum # of e-:
Maximum # of e-:
3d metal complexes with weak field ligands e.g.H2O, NH3and
Cl-belong to this class (class I).
metal
orbitals
(n-1)d
ns
np
Ligand
orbitals
(a1g+eg+t1u)
(a1g+eg+t1u)
t1u*
a1g*
eg*
t2g
D
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MO levels in the absence of pacceptors.
* If is large, eg* can be occupied.
( > pairing energy)
===>
eg* (antibonding): 0 e
t2g(nonbonding): 06e
a1g, t1u, eg(bonding): 12e
Total e-:
Minimum # of e-:
Maximum # of e-:
Complexes of 4d and 5d metals in high oxidation state belong to
this class (class II).
metal
orbitals
(n-1)d
ns
np
Ligandorbitals
(a1g+eg+t1u)
(a1g+eg+t1u)
t1u*
a1g*
eg*
t2g
D
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d0metals:The high-valent d0complexes often have lower
electron counts than 18.
Complexes with bulky ligands:Sterically demanding
ligands will often result in lower than expected electron
counts.
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> 18 electron complexes: Complexes with formally 19 or 20
electrons are known, but they are usually unstable, or
adopt alternate configurations.
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The 18 Electron Rule Is Empirically Justified
The rule is
particularly useful for
Groups 6-8
16 e-Compounds 14 e-Compounds
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The oxidation stateof a metal in a complex is simply the charge thatthe metal would have on the ionic model.
4. Oxidation number of metals and d electron count
e.g.What is the oxidation number of metals in the following complexes?
Fe
H
PR3
PR3
R3PHOs
PPh3
PPh3
ClCl
OCOCWMe6 H
H
d electron count: # of d electrons in the valence shell
= group # - oxidation state.
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Consider complexes W(CO)6and WH6(PMe3)3.
Which one would you expect to have a higher positive charge on W?
b) charge density and formal oxidation states
There is no strict correlation between charge density and
formal oxidation states!Oxidation states in organometallic complexes
are merely formalisms that may bear little resemblance to the actual positivecharge on the metal.
Another example
Any Uses of formal oxidation states?
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Oxidat ion number usual ly can no t be higher than
group member!
==> predict if a compound/intermediate is possible
==> Help to formulate the structure of a compound.
e.g.
(1) Are the following species possible?WMe6 TiMe6 VH7(PMe3)2
(2) WH6(PMe3)3+ HBF4------> [WH7(PMe3)3] BF4Which of the following is unlikely the structure for [WH7(PMe3)3]
+?
W
H H+
H
H
H PP
P
H
H
W
H H+
H
H
H PP
P
H
H
W
H H+
H
H
H PP
P
H
H
(a) (b) (c)
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Co
Cl
NH3
NH3
Cl
H3N
H3N ReH92-
six-coordinated 9-coordinated
5. Coordination number (C.N.) and geometry
a) It is easy to define C.N. for complexes with lone pair donors.
* Monodentate ligand L :
C.N. = # of L present = # of atoms bound to metal
= # of electron pairs involved in M-L sbonds.
e.g.
P l d t t li d
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Polydentate ligands:
C.N. = # of atoms bound to metal
= # of electron pairs involved in M-L sbonds.
# of L present
Co
Cl
CH3
PPh2
Ph2P
Ir
Ph2P
Cl
PPh3Ph3P
H
4-coordinated 6-coordinated
b)For Organometallic compounds, it is difficult to define C.N. e.g.
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Fe Fe
OC COCO
# of L
Fe Fe
OC
CO
CO
# of M-L
FeFe
OCCO
CO
# of e pairs in M-L bond
C.N. = 1
C.N. = 5
C.N. = 3
C.N. = 1
C.N. = 4
C.N. = 2
2 + 3
2
2
+ 3
+ 3
Convention used in this course.
We normally adopt the e- pairs in M-L bonds as # of C. N..
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Further notes on coordination numbers:
a).For TM, C.N. 9 , why?
TM has 9 valence orbitals ((n-1)d, ns, np).
b).dn C.N. and geometry
dn
C.N. geometrical structured6 6 prefer octahedral
d8 4 prefer square planar
d0, and d10 4 prefer tetrahedral
c).Each C.N. is associated with one or more geometries.
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5 Trigonalbipyramidal M
Fe(CO)5
squarepyramidal
M Co(CNPh)52+
6 Octahedron M Mo(CO)6
trigonalprism M
WMe6
(benzonitrile)
d8,,d6
d6,,d7
d6,,d3
d0
6. Effect of complexation
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M-L:
Complexation of L on M may cause:
* the change of electron density distribution on L
* new reactivity of L: unreactive ==> reactive
Examples:
a. Change the electron density on L. donation will reduce electron-density of L.
-accepting will increase electron-density of L.
H2C CH2
Fe
OCOC Fe
OCOC
R
N. R.+ Nu-
but,
+ + R-
Complexation reduce electron-density of olefin
Another example,
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Mn
OC COCO Mn
OC COCO
H
H
H
H
HH
H
H H
H
HH
H
H
E
E
H
HH
H
H
+
+E+ - H
+
Nu-
N. R.
E+
N. R.
but, NaBH4
(H-)
Complexation reduces the e- density on C6H6.
Reactivity towards Nu-: increase
Reactivity towards E+: decrease
Change the electron density distribution on L
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g y
M :N N:
s-donation
:N N: :C O:
= 0 = 0
unreactive
M C OM N NUpon complexation
N N:
M :C O:
C O:
p- backdonation
C ON N
M C OM N N
overallM C OM N N
+ +
Nu- E+
Nu- E+
M M
MM
(e-to on both atom)
(e-to mainly C atom)++
-- - -
7. Differences between metals
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7. Differences between metals
1) Electronegativity differences:
Sc Ti V Cr Mn Fe Co Ni Cu
1.3 1.5 1.6 1.6 1.6 1.8 1.9 1.9 1.9
Y Zr Nb Mo Tc Ru Rh Pd Ag
1.2 1.3 1.6 2.1 1.9 2.2 2.3 2.2 1.9
La Hf Ta W Re Os Ir Pt Au
1.1 1.3 1.5 2.3 1.9 2.2 2.2 2.3 2.5
Moving from left to right, the electronegativity of the elements increases
substantially.
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2). Trend in the stability of high oxidation states.
From left to right decreaseFrom top to bottom increase
e.g. Ti(II) unstable Fe(II) stable
Ti(IV) stable Fe(VIII) not exist
Hf(IV) very stable Os(VIII) stable
Stability of high oxidation states
Early transition metals are electropositive, so they readily lose all their
electrons to give d0
centers (e.g. Zr(IV), Ta(V)). Low-valent early transitionmetals, such as Ti(II) and Ta(III), are easily oxidized.
Late transition metals are more electronegative, thus they prefer lower
oxidation states (i.e., Rh(I) compared to Rh(III)).
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Q ti 2 H ld l i th f ll i t d i th
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Question 2. How would you explain the following trend in theIR dada of (C-O) (in cm-1)?
a). V(CO)6 Fe(CO)5 CO [Ag(CO)]+
1976 2023 2057 2204
b) Cr(CO)6 2000W(CO)6 1998
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Effects of changing net ionic charge, ligands, and metal on the pbasicity of a metal
carbonyl, as measured by (CO) values (cm-1) of the highest frequency band in the IR
spectrum
Changing Metal
V(CO)6
1976
Cr(CO)6
2000
Mn2(CO)10
2000
Fe(CO)5
2023
Co2(CO)8
2044
Ni(CO)4
2057
Changing Net Ionic Charge in an Isoelectronic Series
[Ti(CO)6]2-
1747
[V(CO)6]-
1860
Cr(CO)6
2000
[Mn(CO)6]+
2090
[Fe(CO)6]2+
2204
Replacing p-Acceptor CO groups by Non-p-Acceptor Amines
[Mn(CO)6]+
2090
[(MeNH2)Mn(CO)5]+
2043
[(en)Mn(CO)4]+
2000
[(tren)Mn(CO)3]+
1960
Question 3. Compounds of the formula MH4P3(M = Fe, Ru and Os, P = PR3)
k t h th f ll i t t
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M
H
H
H
H
M
2. e- from d(M) --->s (H2)1. e - from (H2) --->s(M)
H
HM
s
(H2)s*(H2)
Bonding picture of M(H2)
are known to have the following structure.
P Fe H
P
P
H
H H
P Ru H
P
P
H
H H
P Os
H
P
P
H
H H
why not
P Os HP
P
H
H H
?
-
8/10/2019 organologam lengkap
69/70
-
8/10/2019 organologam lengkap
70/70
butadiene cyclooctadiene (COT) cyclooctatetraene (COD) cyclopentadienyl
anion