metal ligand bonding 2008 9

7/21/2019 Metal Ligand Bonding 2008 9

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Metal-Ligand and Metal-Metal Bonding Core Module 4

RED

z

x

y

(n+1)

(n+1)

px py pz

s

n

dxy dxz dyz

d x2-y2

d z2Metal-ligand and metal-metal bonding of the transition metal elementsModule 4

Lecture 1:


2/48

SynopsisRecap of trends of the transition metals. Nomenclature (, h), coordination number and electron counting.

Lecture 2:W

y complexes form. 18-electron rule. Recap of molecular orbital t

eory. s-donor ligand

(hydride complexe

). Con

truction and interpretation of octahedral ML6 molecular orbital energy diagram

Lecture 3:p-acce

tor ligands, synergic bonding, CO, CN

, N2,

Lecture 4:Alkenes and alkynes. Dewar

Duncanson

Chatt model.

Lecture 5:M(H2) vs M(H)2, Mn(O2) com

lexes, O2, NO, PR3.

Lecture 6:p-donor ligands, metal

ligand multi

le bonds, O2

, R2N

, RN2

, N3

. Electron counting revisited.

Lecture 7:ML6 molecular orbital energy diagrams incor

orating p-acce

tor and p-don

r ligands. Relationshi

to s

ectrochemical series, and the trans

effect.

Lecture 8:Bridging ligands, Metal

Metal bonds, d-bon ing.

WorkshopLearning Objectives: by the en

of the course you shoul

be able to

i) use common nomenclature in transition metal chemistry.ii) count valence electrons an

etermine metal oxi

ation state in transition metal complexes. iii) Un

erstan

the physical basis of the 18-electron rule

.iv) appreciate the synergic nature of bon

ing in metal carbonyl complexes.v) un

erstan

the relationship between CO, the 'classic' p

acce

tor and related ligands such asNO, CN, and N2.vi) describe the Dewar

Duncanson Chatt model for metal

alkene and metal

alkynebonding.vii) understand the affect of metal binding on the reactivity of a coordinatedalkene.viii) describe the nature of the interaction between h2-bound diatomic molecules(H2, O2) and t

eir relations

ip to p

acce

tor ligands.ix) describe how H2 (and O2) can react with metal com

lexes to generate metal hydrides and oxides.

x) describe the difference between p

acce

tor and p

donor ligands, and why exce

tions to the18

electron rule occur mainly for the latter.xi) qualitatively describe metal

ligand multi

le bonding xii) understand the origin of the s

ectrochemical series.xiii) calculate bond orders in metal

metal bonding s

ecies, and understand the strengths and limitations of the bond order conce

t.xiv) describe the nature of the quadru

le bond in Re2Cl82

,

articularly the d component, an

triple bon

compoun

s inclu

ing Mo2(NEt2)6.xv)

escribe metal-ligan

an

metal-metal bon

ing using molecular orbital ener


3/48

gy

iagrams.

Bibliography:

Shriver an

Atkins Inorganic Chemistry Ch 8, 9,16.Cotton, Wilkinson, Murillo an

Bochmann A

vance

Inorganic ChemistryCh11, 16Greenwoo

an

Earnshaw Chemistry of the Elements Ch 19, 20-28. Owen an

BrookerA Gui

e to Mo

ern Inorganic Chemistry Mayer an

Nugent Metal-Ligan

Multiple Bon

s

Further rea

ingElectron Counting Huheey, Keiter an

Keiter, Inorganic Chemistry, 4th E

pages625-630.Bon

ing Murrell, Kettle an

Te

er The Chemical Bon

Tetrahe

ron, 1982, 38, 1339. H2 Angew. Chem. Int.E

., Engl. 1993, 32, 789.O2 Chem. Rev. 1994, 3 (various articles)

Associate

Courses

AKDK Transition metal chemistry1st year VC Structure an

bon

ing1st year JEM Molecular orbitals

1st year RNP Group theory2n

year KWSurface Chemistry 2n

year JML Transition metal organometallics2n year SBD Inorganic mechanisms I

2n

year MCRC Vibrational spectroscopy2n

yearMCRC Photoelectron spectroscopy2n

year AKDK Main group clusters an

organometallics3r

year RED Inorganic materials chemistry

3r

year RED Inorganic mechanisms II3r

yearCatalysis option mo

uleWhy is metal ligan

bon

ing important?

Catalysts e.g. polymers, pharmaceuticals, bulk chemicals

Ti Cl Ph3PCl Ph3P

PPh3

Rh H

[(h5- C5H5)2TiCl2[R

(H)(PP

3)3]


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Bioc

emistry e.g. oxygen transport, p

otosynt

esis, enzymes, medicines, poisons

proteinHN O

N OproteinHN O

N ON Fe N oxygen binding

N Fe N

N NO-

N O N OO-

O O

`Organic' c

emistry met

odology e.g. M(CO)3 arenes, Pd catalysed C-X (X = C, N, S, O) bond formation, metat

esis.

en

anced nucleop

ilic substitutionen

anced solvolysis

X X

en

anced acidity HH Hsteric

indranceCrOC CO CO

en

anced acidity

R

R-CrOC CO COT

is course is primarily concerned wit

t

e transition metals (`d-block' metals).

Recap


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Important trends:

1. Radius (Covalent/ionic) :- Increases from rig

t to left and down a group.

2. Electropositivity:- electropositive c

aracter increases from rig

t to left and down a group.

T

e trends observed in 1 and 2 are a result of t

e effective nuclear c

arge (Zeff) t

at is a consequence of s

ielding and penetration. s > p> d > f

T

e relatively very poor s

ielding of an electron in an f-orbital results in a steady decrease in t

e radii of t

e lant

anides (approximately 25%). T

is is known as t

e lant

anide contraction. Wit

respect to t

e transition metals t

e resul

t is t

at t

e radii of t

e 2nd and 3rd row transition metals are very similar.E.g. Co(III) (0.55), R

(III) (0.67), Ir(III) (0.68). T

is

as repercussions inmetal- ligand bonding and

ence c

emical properties. In general w

en descendinga group t

e 1st row transition metal is distinct in terms of its bonding and properties from t

e 2nd and 3rd row metals.

3. Variety in oxidation state:- earlier metals (group 4 to 7) ex

ibit t

e greatest variety in oxidation state. Hig

er oxidation states more commonly observed for 2nd and 3rd row metals.

e.g. Fe(III), Ru(VIII), Os (VIII).

Ionic vs covalent bonding

T

e 3d orbitals in t

e first row metals are not as diffuse as t

e 2nd and 3rd row 4d and 5d orbitals. T

is leads to a larger ionic component in t

e bonding of first row metal complexes. However in many cases t

e bonding in 3d metals can bedescribed using covalent t

eories suc

as molecular orbital t

eory.Compare t

is to t

e 4f orbitals of t

e lant

anides t

at are essentially coreorbitals and cannotparticipate significantly in covalent bonding. T

e bonding in lant

anide complexes can be considered almost totally ionic and t

ey are often considered to be more similar to t

e alkaline eart

metals t

an t

e transition metals.Nomenclature and electron counting


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h

apticity t

e number of atoms of a ligand attac

ed to a metal.

O

O O O

M MM

h1-O2 h2-O2h6-C6H6

T

e number of metal atoms bridged by a ligand

O O

C C

M M

M M M

2-CO3-CO

e.g.

MO OM MO OMh2, 2 - O

2 2 - O2h1 taken as defaultMetal oxidation state

Oxidation state C

arge on t

e- Sum of t

e c

arges= complexof t

e ligands


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8/48

+R

RC(O)n H+

R H

RC(O)Hn=1

Electroneutrality principle

T

e electronic structure of substances is suc

to cause eac

atom to

ave essentially zero resultant c

arge. No atom will

ave an actual c

arge greater t

an

1.i.e. t

e formal c

arge is not t

e actual c

arge distribution.e.g. P

otoelectron spectroscopy (PES) is a tec

nique t

at allows t

e experimental determination of orbital energies.IrH3C CH3H3C CH3Ir

H3C DMSO

CH3IrOC CO

Ir(V) Ir(III)Ir(I)

PES s

ows t

at all t

ree iridium complexes

ave similar d-orbital energies indicating t

at t

e formal oxidation state is not t

e actual c

arge on t

e metal.

d-electron count =group number- oxidation state

Electron Counting

Total Valence =Electron Countd-electron +countelectrons donated +by t

e ligandsnumber ofmetal-metal bonds

(ignore overall c

arge on complex)


9/48

T

ere are two met

ods t

at are commonly used and it is very important to avoid confusion.

M-L

M-LM. + L. neutral (or radical) formalism

M+ + L- ionic formalism

To avoid confusion we will use t

e ionic formalism to determine t

e total number of valence electrons (electron count). However for some ligands O2, NO and organometallics (carbenes, carbynes) t

e neutral formalism is more appropriate.

Number of electrons donated by eac

ligand (using ionic formalism)2e CO, RCN, NR3 (amines), PR3 (p

osp

ines), N2, O2, C2R4 (alkenes), H2O, H-, CH3- (or any alkyl or aryl group, R), F-, Cl-, Br-, I-, CN-, NR2- (bent), (h1-C5H5)-4e R2PCH2CH2PR2 (bis-p

osp

ines), h4-dienes, NR2-(linear), (CH3CO2)-, NR2-(bent),O2- (double bond), S2-6e (h5-C5H5)-, h6-C6H6, NR2- (linear), O2- (triple bond), N3-, P3-Metal-metal bonds

Single bond counts 1 per metal

Double bond counts 2

Triple bond counts 3

Quadruple bond counts 4Metal metal bonding is more common for 2nd and 3rd row metals t

an for 1st row.

e.g.

Cr(CO)6 : 6 + (6 x 2) = 18 [Co(NH3)6]3+ : 6 + (6 x 2) = 18

[CoCl6]3- : 6 + (6 x 2) = 18 PtBr2(PP

3)2 : 8 + (2 x 2) + (2 x 2) = 16


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CO OC CO CO

c.f.

Cl ClCO Mn Mn COCOCO CO CO

7 valence electrons eac

.Electron s

aring gives a count of 8 per Cl

Mn2(CO)10per Mn: 7 + (5 x 2) + 1 = 18

Coordination numbermetal oxidation stated-electron count total valence electronsCr(CO)6 6

0 6 18 [Co(NH3)6]3+6 III6 18

[CoCl6]3- 6III 6 18

PtBr2(PP

3)2 4II 8 16

R

(CO)(H)(PP

3)3 5 I8 18

TiCl4 4

IV 0 8

Cr(h6-C6H6)2 6 06 18

Fe(h5-C5H5)2 6 II 6 18 [ReOCl5]-

6 VI1 15 (17)

W

y complexes form

(T

ermodynamic stability of transition metal complexes)

1. T

e number and strengt

of metal-ligand bonds.T

e greater t

e number of ligands, and t

e stronger t

e bonds, t

e greater t

e t

ermodynamic stability of t

e resulting complex. i.e. in general t

e more ligands t

e better. Larger metals can accommodate more ligands. In general coordination numbers are greater for t

e earlier transition metals (groups 4 7) compared tot

e later ones. Coordination numbers for lant

anide complexesare generally

ig

er t

an for transition metals. d8 square planar complexes arestable because 4strong bonds are collectively stronger t

an 6 bonds t

at would be collectively


11/48

weaker for t

is electron configuration.

2. Steric factors.T

e number of ligands is limited by ligand ligand repulsion. T

e size of metalsand common ligands leads to transition metals generally accommodating a maximumof six ligands

ence t

e vast number of 6 coordinate transition metal complexes. For similar reasons t

ere are many 9 coordinate lant

anide complexes.

3. T

e c

arge on t

e complex.Large positive and negative c

arges cannot easily be supported. Continually removing electrons from a complex will result in increasingly large ionisation energies, and increasing t

e number of electrons will lead to large electron-electronrepulsive forces.

4. T

e electronic configuration.Crystal field stabilisation energy, Ja

n-Teller distortion.

Free ion Mn+ + 6L

Eiii)

i)iv)

ii)

i) electrostatic attractionii) destabilisationof core electronsiii) destabilisation of valence electrons iv) CFSE

Note t

at crystal field stabilisation energy (CFSE) contributes only approximately 10% to t

e overall t

ermodynamic stability.Recap of molecular orbital t

eory

a) Orbitals must be of appropriate symmetry b) Orbitals must overlapc) Orbitals s

ould be of similar energyb) and c) determine t

e energy of t

e interaction. T

e interaction energy is stronger for orbitals t

at

ave good overlap and are close in energy.

W

en t

e MOs are made up of 2 component orbitals of different energies


12/48

T

e bonding orbital looks more like t

e lower energy component

T

e antibonding orbital looks more like t

e

ig

er energy component

Electronic configuration: Transition metal valence orbitals and t

e 18 electronrule

Valence s

ell of transition metals nd + (n+1)s + (n+1)p orbitals (w

ere n = 3-5).5 + 1 + 3 = 9 orbitals. Two electrons per orbital = 18 electrons.(Just a restatement of t

e Lewis octet rule wit

extra 10 d-electrons)

For Met

ane

n antibonding orbitals

2px 2py 2pz

2 sn ligand orbitals

z

x n bonding yorbitals

H

C C H4HH HFor a transition metal complexz

x n antibonding y

orbitals


13/48

(n+1)

px py pz

(n+1)s

9-n non-bonding orbitals

n dxy dxz dyz d x2-y2d z2

n ligand orbitals

n bonding orbitalsLL LM M L6L LL

For many complexes an electronic configuration of 18 valence electrons is t

e most t

ermodynamically stable, especially for diamagnetic organometallic complexes,

owever as noted earlier t

e electronic configuration is only one factor t

atcontributes to t

e overall t

ermodynamic stability of a complex. T

ere are manyimportant exceptions to t

e 18 electron rule including:

1st row coordination complexes w

ere t

e bonding is predominantly ionic. square planar d8 complexes (16 e-). early metal complexes wit

p

donor ligands.

aramagnetic com

lexes.

Ligand classification

Metal

ligand bonding can be divided into three basic classes

1. s-donor

e.g. H, CH3 (or any alkyl or aryl group, R), H2O, NH3, NR2 (bent)

2. s -donor, p

acce

tor (sometimes referred to as `p

acce

tors' or `p

acids')


14/48


15/48


16/48

bo

di

g molecular orbital

s sa 1eg orbital s

1eg

1t1u

1a1g

LL

t1ua1g

e.g. 6 NH3 lone pair orbital

LLM L M L LL L LL Lmetal in Oh field ligand in Oharrangement

Note that there are no linear combination

of ligand orbital

that have t2g

ymmetry. Therefore the t2g orbital

are non-bonding and completely metal ba

ed. The 2eg orbital

are sand have ligand character but are approximately 80% metal ba

ed (remember the antibonding orbital i

mainly of higher energy

tarting orbital character). When we talk about

plitting of metal `d-orbital

' in cry

tal field theory we are ignoring the ligand character that i

pre

ent in

ome of the `d-orbital

', however it i

till a good fir

t approximation and the relative energie

between d-orbital

are correct. We will

ee that when we include p-acce

torsa

d p-do

ors that the t2g orbitals are

o lo

ger

ure metal orbitals but also co

tai

some liga

d character.Notes o

molecular orbital diagrams

1. The total

umber of molecular orbitals should be the sum of the

umber of

recursor orbitals.

2. O

ly orbitals of the same symmetry ca

i

teract a

d the resulti

g molecular orbitals will have the same symmetry as the

recursor orbitals

3. Where do the a1g, eg, t1u li

ear combi

atio

s of atomic orbitals come from? Usi

g grou

theory it is

ossible to determi

e the symmetry of the orbitals i

volved. i) determi

e the

oi

t grou

of the molecule (i

this case Oh).ii) treat the liga

d orbitals (i

this case s) a

a

ingle entity and apply each


17/48

ymmetry element of the point group noting how many of the individual orbital

move under each operation. Thi

i

the reducible repre

entation.iii) determine which character

um to the reducible repre

entation thu

obtaining the irreducible repre

entation. (in thi

ca

e for the octahedral array of s-H orbital

it will be a1g + eg + t1u).iv) repeat for the 3 x p and 5 x d orbital

(the 1 x

can be read off directlya

having a1g

ymmetry)or alternatively look at the right hand portion of the group table and read offthe orbital

ymmetrie

.v) apply projection operator

to determine the linear combination

of orbital

4. The origin of

ymmetry label

nxyzApart from being character

in group table

the label

can be u

ed to de

cribethe

ymmetry oforbital

.n = orbital

of the

ame

ymmetry are numbered

ucce

ively in order of increa

ing energy x = a if

ingly degenerate and

ymmetrical to C2n rotation about the principle rotation axi

x = b if

ingly degenerate and un

ymmetrical to C2n rotation about the principlerotation axi

x = e if doubly degeneratex = t if triply degeneratey = 1 if

ymmetrical to reflection through a reference mirror planey = 2 if un

ymmetrical to reflection through a reference mirror plane z =

nothi

ng

if there i

no inver

ion centrez = g if

ymmetrical to inver

ionz = u if un

ymmetrical to inver

ion

5. What group theory cannot tell u

.i) What the orbital

look likei) The energy of the orbital

and the magnitude of the precur

or orbital

interactionRecap of cry

tal field

plitting diagram

By con

idering the repul

ive interaction

between electron

it i

po

ible to qualitatively determine the ordering of metal d-orbital

. Cry

tal field theory i

a purely electro

tatic approach. Here the d- orbital

are pure. Compare the diag

ram below and `d-orbital

' of MO diagram above for octahedral complexe

.

z

x

y

d x2-y2


18/48

dx2-y2d z2

Barycentre

dxy dxz

dyzdxy

dxy dxz dyz dx2-y2 d z2

d z2dxz dyz

s-donor, p-acce

tor (`p-acce

tors' or `p-acids')

These i

clude: CO, CN, NO(li

ear), H2, C2H4, N2, O2, PR3, CR2

We ca

view the metal-liga

d bo

di

g as a s-donor interaction (

ame a

for H) withan additional p- i teractio that arises from overla

betwee

metal-based orbitals a

d em

ty orbitals o

the liga

d that ca

acce

t electro

de

sity.

Metal com

lexes of CO are a good exam

le. e.g. Some of the bi

ary metal carbo

yls

Grou

5 6 7 8 9

10

V(CO)6 Cr(CO)6 M

2(CO)10 Fe(CO)5

Fe2(CO)9Co2(CO)8 Ni(CO)4

Mo(CO)6 Tc2(CO)10 Ru(CO)5

Ru2(CO)9

W(CO)6 Re2(CO)10 Os(CO)5

Os2(CO)9Rh2(CO)8

Ir2(CO)8


19/48

Some structures

CO COCO CO

CO OC CO CO CO COCO VCO COCr CO COCOM

M

CO CO FeCOCO CONi CO COCO COCO COCO CO CO CO

CO COCO CO

CO CO CO

Co CoCO COCO Co Co COCO CO

CO CO CO

Note: whe

cou

ti

g electro

s CO always co

tributes 2 electro

s whether termi

alor bridgi

g

MO diagram of CO

C CO

x y 6sz 2p

2

5s

2

1pO

6su(s)

2p


20/48

5s2p

CO molecular orbital

1pCO molecularorbitals 4s 4s2

3s

1

2s1s 1

HOMO 5s orbital i

lightly antibonding and ha

ignificant C 2

character. Thi

i

whyCO bond

to a metal a

a s-donor through the C atom and not the O atom (better overlap).

In can be

een that the 2 x 2p LUMO orbitals (a tibo di g) are em

ty. It is theseorbitals that ca

i

teract with metal d-orbitals acce

ti

g electro

de

sity.

Directio

of charge tra

sfer

s-donorM C O

5s

Direction of charge tran

fer

M C O

p-acce

tor2pThe s-donor interaction increa

e

the electron den

ity on the metal anddecrea

e

the electron den

ity on the CO ligand.The p-acce

tor i

teractio

decreases the electro

de

sity o

the metal a

d i

creases the electro

de

sity o

the CO liga

d.Both effects `rei

force' each other. Sometimes referred to as sy

ergic bo

di

g.

p -acce

tor liga

ds such as CO ca

relieve

egative charge build-u

at a metal ce


21/48

tre. e.g. stabilise com

lexes with metals i

a low formal oxidatio

state.

Ex

erime

tal evide

ce for bo

di

g model

IR a

d Rama

s

ectrosco

y a

d si

gle crystal X-ray diffractio

.

Characterisatio

of metal carbo

yls

O

CO O C CCrC O

As n(C-O) decreases C-O bo

di

g decreases a

d M-C p-bo

di

gi

creasesO C C O C

O

M C O

M C OCr-C = 195.5

mC-O = 114.0

mn(C-O) = 1984 cm-1

Tre

ds i

n(CO)

C-O = 112.8

mn(C-O) = 2143 cm-1

a) isoelectro

ic series b) as CO liga

ds are lost c)as other liga

ds cha

ge

n(CO) cm-1

(T1u)

n(CO) cm-1 n(CO) cm-1

M

(CO) +2094 Mo(CO)6 1987 Ni(CO)(PF3)3

2073

Cr(CO)6 1984 Mo(CO)5 1966

Ni(CO)(PCl3)3 2059-V(CO)6Ti(CO) 2-1845 Mo(CO)4 1944,1887 Ni(CO)(PMe3)3 1923

1750 Mo(CO)3 1862

d) coordi

atio

mode


22/48

Free Termi

al 2-CO 3-CO

O O C C

C M M M M M MnCO (cm-1) 2143 1850-2120 1750-1850 1620-1730

Always thi

k i

terms of CO liga

ds com

eti

g for whatever electro

s are available o

the metal!No

-classical carbo

yls

Pd(CO) 2+ Pt(CO) 2+ Ag(CO) + Au(CO) +Hg(CO) 2+

n(CO)/cm-1

2248

2244

2200

2217

2278

I

these com

lexes electro

de

sity is

ot tra

sferred from the metal tothe liga

d p- acce

ti

g orbitals. The major i

teractio

is s-donation from the CO 5s (anti-bonding) orbital to the metal. Therefore the CO

tretching frequency i

> free CO.

Similar p-acce

tor ligands

Other ligands that are ex

ected to exhibit very similar bonding to CO are the isoelectronic ligands CN

and NO+. (We will see later that NO can also coordinate in an alternative terminal mode).N2 is also isoelectronic with CO.

MO diagram of N2

N N2

x 3su(s)

y

z 1pu(p)

2

3sg(s)N

3su(s)


23/48

2p

3sg(s)

1pu(p)

1pg(p)1pg(p)

2s 2su(s) 2

2sg(s)

Compare HOMO 5s (s*) of CO and HOMO 3sg (s) of N2. Coordination of N2 decrea

e

N-N bon

trength.N2 can act a

a p-acce

tor usi

g LUMO 1pu same as for CO.Very few metal com

lexes of N2 com

ared to CO.A

other reaso

is that the e

ergy differe

ce betwee

metal d-orbitals a

d the 3sg

orbital ofN2 i

greater than that for metal d-orbital

and the 5s orbital of CO. (remember the clo

er inenergy the precur

or orbital

are, the

tronger the bond). Therefore M-N2s-bonds areweaker than M

CO s-bonds. For similar reasons N2 is also a

oorer p-acce

tor ligand thaCO.

Other p-acce

ting ligands

Im

ortant exam

les include O2, H2, PR3 and alkenes.

Com

lexes of dioxygen

OO O O O

O

N NFe N Ti NN N NN O


24/48

haem

O O

O O CrO Ti(h2-O2)(porp yrin)

N O N Cu CuN NO O N O

N O OCr(h2-O2)4

aemocyanin

h1 vs h2 bonding in O2 complexes

MO diagram of O2

O O2x 3su(s)yz 1pg(p)

O

3su(s)

1pg(p)

2

2

1pu(p)

3sg(s)

1pu(p)

3sg(s)

2su(s)

2

2

2sg(s)h1 - bondingDirection of c

arge transfer

O M O

s-donor

1pgx

Directio

of charge tra

sferO


25/48

M Op-acce

tor

1pgy

h2 - bonding (more difficult)or

2+ 2-M + O2

M+ O2Electron transfer first 1pgy

ow full.Directio

of charge tra

sferDirectio

of charge tra

sfer

MOO 1pgxMO1pgx

O

Directio

of charge tra

sfer

M(

o back-do

atio

as

acce

ti

g

orbitals are full) O2 is very oxidisi

g. Thi

kof as electro

tra

sfer2- 2+OO 1pgyfollowed by bi

di

g of O2to M. Theory

suggests this may be best o

tio

.

Characterisatio

what is the oxidatio

state of O2?

I

a

y give

com

lex, all we k

ow for sure is that the O2 molecule is bo

ded tothe metal. Neutral dioxyge

, su

eroxide (O2-) a

d

eroxide (O22-) are all well k

ow

forms of the

O2

u

it, so a

y give

com

lex could be {M-O2}, {M+-O2-} or{M2+-O22-}

3su(s)

1pg(p)


26/48

1pu(p)

3sg(s)

2-O2 O2 2

Compari

on of MO diagram

of dioxygen,

uperoxide, and peroxideVibrational frequencie

and O-O bond length

r(O-O) / pm n(O-O) / cm-1

+ -O2 (AsF6 ) 122 1858

O2 1211555

- +O2 (K ) 133 11462- +O2 (Na )2 149

842

h1-O2 115-1301130-1195

h2-O2 130-152800-930

As t

e electron density in t

e porbitals i

creases the O-O dista

ce i

creasesa

d the vibratio

al freque

cy decreases

What ha

e

s if the s* (3su) orbital become

occupied?

(But)3SiO

2 Ta

(But)3SiO

OSi(tBu)3 + O OO

2 Ta(But)3SiO

OSi(tBu)3


27/48

tOSi( Bu)3

Ta{OSi(tBu)3}3

O O

{(tBu)3SiO}3Ta

The Ta complex i

reducing and ha

two electron

in a high-energy orbital HOMO.The Ta complexe

have orbital

of the correct

ymmetry and can donate 4 electron

to a molecule of O2 occupying 1pg a

d 3su of O2 cau

ing cleavage of the O2 bond.

Why i

h1-O2 bent w

en CO is linear?

Simply because O2

as to accommodate an extra pair of electrons in t

e 1pg (p) orbital. These occu

y 1pgx (to form the s-bond through one lobe of the 1pgx orbital) leavi g 1pgy to form a p-acce

tor i

teractio

.NO revisited

NO ty

ically ado

ts o

e of two termi

al coordi

atio

modes (be

t a

d li

ear)

CO

CO

M

N OCO

NH3NH3

NH3

Co

2+ NH3N

O

N


28/48

Cl Ru N O

CO NH3O Ph3P PPh3

Characterisatio

M-N-O a

gle/ n(N-O)/cm-1

Fe(CN)5(NO)2- 178 1935

M

(CN)5(NO)3- 1741700

Co(NH3)5(NO)2+ 1191610

CoCl(e

)2(NO)+ 1241611

How ma

y electro

s does NO do

ate? Li

ear:i) 1 electro

goes from NO to the metal, givi

g NO+ + M-.

ii) NO+ is the

isolectro

ic with CO, a

d do

ates 2 electro

s from NO to metal

2+1 = 3, so NO is a 3-electro

do

or. Be

t:i) 1 electro

goes from metal to NO, givi

g NO- + M+.ii) NO- is the

isolectro

ic with O2, a

d do

ates 2 electro

s from NO to metal

-1 + 2 = 1, so NO is a 1-electro

do

or

Strategy for determi

i

g be

t or li

ear, electro

cou

t a

d oxidatio

state:1) Remove NO (

eutral) from com

lex a

d calculate electro

cou

t a

d oxidatio

state of remai

i

g fragme

t.2) Add 1 or 3 electro

s

er NO to i

crease electro

cou

t to 18 (or as close as

ossible without exceedi

g 18). You

ow have the total electro

cou

t at the me

tal a

d the M- NO geometry.3) Determi

e the metal oxidatio

state of the com

lex i

cludi

g the NO liga

d(s) a

d co

sider li

ear NO to be NO+ a

d be

t NO to be NO-.

e.g.

total vale

ce electro

smetal oxidatio

state d-electro

cou

tM

(CO)4(NO) 18 -I8

Co(NH3)5(NO)2+ 18 III6

RuCl(NO)2(PPh3)2 17 I

7Com

lexes of dihydroge

(H-H = 74.1

m i

H2)

82

mH HCO111

mH H

i PiPr3


29/48

160

mH HP(Pri)3

COW P(iPr)3

COP Pr3 Ir

H

Cl

Cl PPh3

COIr PPh3

Cl

H H2H

xy 1su(s)

z

1su(s)

1

1

1sg(s)

1sg(s)


fer

M

s-donor

H1sg(s)H


30/48


fer

M

p-acce

tor

H1su(s)H

Note: the s and s* orbital

of H2 perform the

ame role

a

the s and p* orbitals i

CO

The a

tibo

di

g s* H2 orbital i

of p-symmetry about a

axis

er

e

dicular to the H-Hbo

d a

d ca

i

teract with a metal orbital of p-symmetry.

If sufficie

t electro

de

sity is tra

sferred from the metal to the s* orbital ofH2 the H-Hs-bond will break and give two M

H (metal

hydride) s-bonds (oxidative addition).

e.g.

Ph3P OC

R Rh PPh3

+ H2

oxidative addition

Ph3P OC

HR Rh H


31/48

PPh3

RhIRhIII

Characterisation Dihydrogen M(H2) or dihydride M(H)2 com

lex ?

Technique h2 H-H di

ydride

Neutron diffraction H-H~82 pm H-H~160 pm

NMR Low field, JHD~30Hz Hig

field, JHD~5Hz

IR n(H-H) ~ 3000 cm-1

n(M-H) v. low

n(M-H)~2150-1750 cm-1

Alke

es

p-acce

tor liga

ds. Alke

e com

lexes form basis of ma

y catalytic reactio

s e.g.

olymerisatio

, hydroge

atio

a

d metathesis.

Com

lexes of alke

es

+

PCy3 PhH RMe Ph3P Zr Ph3PCOCl

Ru PhCl

s

p

H

H

sH s s

Hp

s

H H H H


32/48

s-donorH H CAlkenep-orbitalCM H H

Directio

of charge tra

sfer

p-acce

torH H CAlke

ep*-orbitalCM H H

Directio

of charge tra

sfer

H H H H HH

H H

M M

Dy

amics i

alke

e com

lexes

RhH H

H H H H

H H

Rh

Ha2


33/48

Hb2

Ha1

Hb1Rh

Hb1

Ha1

Hb2

Ha2What determi

es e

ergy barrier to rotatio

?1. s-bonding (dominant) interaction i

not affected by rotation (no change inoverlap)2. p-bo

di

g (mi

or) is broke

, but other

ote

tial bo

di

g orbitals at 90o to start

oi

t hel

lower activatio

e

ergy.

PR3 com

lexesPR3 ca

also act as p-acce

tor liga

ds. I

this case the orbitals are usually

hos

horus s*orbital

. Complexe

of PR3 ligand

are very important cataly

t

for many reaction

.

PR3 ligand can tabili e low oxidation tate by p-acce tor i teractio s a d highoxidatio

states by stro

g s-donation.

RP M RR

p-acce

tor i

teractio

P sorbital


fer

Cataly

i

example

Enantio

elective

ynthe

i

of S-DOPA

Ph2 +P SolventRhP Ph2SolventPh NHC(O)Me+ H2


34/48

CO2Me

Solvent = MeOH

Ph NHC(O)Me

H CO2MeS-DOPA precur

orTreatment of Parkin

on'

Di

ea

e

CO

Ph3P Rh

PPh3PPh3C7H15

+ H2 + COH C7H15 O

linear aldehyde

Lot

of bulk chemical u

e

, e.g. Perfume

, agrochemical

The Rh-L bonding of Rh-CO, Rh-H2 (Rh-(H)2), Rh-alkene, and Rh-PR3 all play anintegral role in thi , and many other, catalytic reaction .s-donor, p-do or (`p-do ors')

Liga

ds that fall i

to this category i

clude: F, Cl, Br, I, O, OR, S, SR, N, NR2(li

ear), NR (be

t a

d li

ear), P.We ca

view the metal-liga

d bo

di

g as a s-donor interaction (

ame a

for H) with an additional p- i teractio that arises from overla

betwee

metal-based orbitals a

d full orbitals o

the liga

d that ca

do

ate electro

de

sity.

Directio

of charge tra

sfer

M

s-donor


fer

M

p-do

or

x

Ma

d /or

y


35/48


36/48

Mo PhMe2P COO O ClOPhMe2P CO Cl18 e- 16e- 18 e-X-ray shows verydouble bo

dshort W-O dista

ce

Metal oxides are used as source of oxyge

for the oxidatio

of orga

ic com

ou

dse.g. catalytic e

oxidatio

of alke

es.

O

N N M

O O OR

RNaOCl(bleach)

Other commo

multi

le bo

ds are the amido (NR2-), imido (NR2-) a

d

itrido (N3-)liga

ds.

R - + RN R M N M

NM R

2 electro

do

or

R4 electro

do

or

R

4 electro

do

or

R R R

N N N +

M M M -

4 electro

do

or6 electro

do

or6 electro

do

or

TaCl5 + xs tBuNH2

tBu


37/48

H

N NtButBu

NH2ClTaClCl

Cl

Ta

NH2 tBu

tBu

N N

H

tBu

Ca

we use N2 as a source of

itroge

i

orga

ic chemistry?

tBuN

Ph2But

N Mo

NPhtBuN

Ph

+ N N


38/48

2 Ph

MoN

tBu N Ph

N PhtBu

tBu

N

N NM

+ Ar

(CF3CO)2

OH

NHCOCFO O

Ar 3

Electro

cou

ti

g of p-do

or com

lexes ca

be difficult. As a rule of thumb i

voke as ma

y multi

le bo

ds as

ossible to get as close to (but

ot over) 18.

total vale

ce electro

smetal oxidatio

stated-electro

cou

t(tBuO)3WN 12 (18) VI 0

(h-C5H5)2V(NP

) 17 IV 1

ReMe4(O) 13(15) VI 1W

at effect do p-acce

tors a

d p-do

ors have o

the chemistry of metal com

lexes?MO diagram of Oh com

lex with p-do

or liga

ds


39/48

t1us3t1u

s2a

e.g. [CoCl6]3-z

x

yligand t2g

a1gs 2eg

pt2ggoct

t2gs2t

n.b. t1gn.b. t2u

t1g t2u t1u t2g eg

Mainly ligand in character. Ignored in electron counting of metal valence electron

.3-p t2gt1u

[CoCl6]is co

sidered 18

s 1eg

s 1t1u

a1gelectron, even though 42


40/48

electron

in MO diagram.

s 1a1g

L LL LM L M L

L L L LL L

metal in Oh field ligand

in Oh arrangement

Note the effect on the t2g d-orbital

in compari

on to the s-only ca

e. The

e t2gorbital

have ri

en in energy, clo

er to the eg level, re

ulting in a reductionof oct (10 Dq).MO diagram of Oh com

lex with p-acce

tor liga

ds

3-p 3t1ue.g. [Co(CN)6]zst2g

x

t1u

n.b. n.b.

t1gt2u

t1g y t2u t1us2a1g

t2g

ligand t2g

a1gp 2t1us 2eg

eg

t2g


41/48

p t2goct

s 1egeg t1u a1g

s 1t1u

s 1a1g

L LL LM L M LL L L LL Lmetal in Oh field ligand

in Oh

arrangement

Note the effect on the t2g d-orbital in compari on to the s-only ca e. The t2g ha

been lowered in energy with re

pect to the eg level re

ulting in an increa

e in oct (10 Dq).Summaryeg

eg eg

oct

t2g

t2gt2gs-onlyp-do

orp-accetor oct p-accetor > s-only > p-do or

p-do

ors a

d the 18-electro

rulep-acce

tor liga

ds usually obey the 18-electro

rule, those with p-do

ors do

ot

ecessarily do so.For p-do

or liga

ds the metal t2g orbitals are

ow slightly a

tibo

di

g (p*) therefore it is less e

ergetically favourable to fill them.3-e.g. CrCl6with 15 total vale

ce electro

s is stable.


42/48

S

ectrochemical series

The s

ectrochemical series is a list of liga

ds i

order of i

creasi

g liga

d field stre

gth. Electrostatic model ca

ot accou

t for the order.

CO > CN- > PPh3 > NH3 > H2O > OH- > F- > Cl- > S2- > Br- > I-

p-acce

tor s-only p-do

or

I

creasi

g oct

oct i

creases with i

creasi

g p-acidity of the liga

ds e.g. Field stre

gth determi

e s

i

state of metal com

lexes[CrCl6]4- [Cr(CN)6]4-

High s

i

Low s

i

Tra

s-effect a

d Tra

s-i

flue

ce

These

he

ome

a will be discussed i

more detail later i

I

orga

ic Mecha

isms I. The tra

s- effect a

d tra

s-i

flue

ce hel

to ratio

alise the stability a

d substitutio

chemistry of tra

sitio

metal com

lexes,

articularly square

la

ar P

d a

d Pt com

lexes.

The tra

s effect is a ki

etic

he

ome

o

a

d describes the i

flue

ce of a

o

-labile grou

o

the rate of substitutio

of a liga

d tra

s to it.

CO, CN- > PPh3 > NO2- > I- > Br-, Cl- > NH3, OH-, H2O

p-acce

tor p-do

or s-onlye.g.

NH3

Cl- NO -

NH3 -PtCl 2-


43/48

Cl Pt

Cl

ClNH3

2 -2 Cl Pt

Cl

ClNO2

-2- NO2- NH3PtCl4Cl Pt

ClNO2H3N Pt

ClNO2Metal-metal bonding

Complexe

with metal-metal bond

Cl Cl 2-Cl Cl

3- Cl Cl

CO OC CO CO

iPr

iPrRe ReCl Cl


44/48

W WCOMn MnCO iPr

iPr

Cr Cr

iPr iPrClCl Cl Cl

Cl Cl Cl ClCOCO CO COiPr

quadruple triple

ingle

Bonding in `Bare' M2 dimer

(e.g. V2)

x

quintuple?

iPry sz

p

dd orbital

dd

xypp

dxy dx2-y2 dx2-y2

s

V V2 Vconfiguration s2p4d4


45/48


46/48

x2-y2

d

xy

d

xy

d

xy

z2

xz

yz

p

dxz

dxzp

dyz dyz

L L ML L L s

L L L L L L M M MLL L L L

dz2sdz2

Configuration rM-M /Orientation of pm ML4 unit


47/48

Re2Cl 2-

2-s2p4d2 222 Eclipse

Staggere

Os2Cl8s2p4d2d*2 218or eclipse

Triple bon

s

Cl Cl3- L LCl L LNMe2NMe2NMe2Cl ClW WM M

L L

NMe2Mo Mo

NMe2NMe2Cl Cl Cl ClBri ge Non-bri ge (eclipse )Non-bri

ge

(staggere

)

note: 2

orbitals per metal are now 'factore

out'

L L L L

L LLL M L M M L L

M MLL

"

x2-y2" "

xy"L L L d

dx

ysL LNon-bridged (

taggered)

pz

s


48/48

metal ligand bonding 2008 9

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