Transition Metal ComplexesBonding

Bonding Theory

•Crystal Field TheoryCoulomb Interactions–attraction between metal ion and ligand electrons–repulsion between metal electrons and ligand electrons

•Ligand Field TheoryMO TheoryOverlap of orbitals between ligands and metal

Crystal Field Theory

n+L M

repulsion

attractione-

e-

Crystal Field Theory

Free M ion andLigands

M andLigands

attraction

repulsion

E

n+L Mrepulsion

attractione-

e-

Splitting of d Orbitals in Oh Field

o: Ligand Field Splitting Parameter (10 Dq)

More repulsion

Less repulsion

Factors Affecting the MagnitudeCrystal Field Splitting

•Identity of the ligand•Charge on the metal•Position in a group•Geometry and coordination number

Absorption Spectrum of [Ti(OH2)6]3+

eg t2g

Variation of o in Octahedral Ti(III)Complexes

•Ti(III) is a d1 complex and exhibits one absorption inits electronic spectrum due to transition of theelectron from the t2g orbitals to the eg orbitals. Theenergy of the absorption corresponds to o.

Spectrochemical Series:Arrangement of ligandsin order of increasingligand field strength

Spectrochemical Series•Order of ligand field strength with decreasing o:

acceptor > no effect > weak donor > donorstrong field weak field

CO > CN–> PPh3 > phen ~ NO2–> bipy > en > NH3 ~ py

> CH3CN > NCS- > H2O > C2O42–> OH–> F–> N3

- >NO3

–> Cl–> SCN–> S2- > Br–> I–

•Order of metal with increasing o:Increases with increasing oxidation numberIncreases down a groupMn(II) < Ni(II) < Co(II) < Fe(II) < V(II) < Fe(III) < Co(III) <Mn(IV) < Mo(III) < Rh(III) < Ru(III) < Pd(IV) < Ir(III) <Pt(IV) weak field strong field

Ligand Field StabilizationEnergy (LFSE)

LFSE = (- 0.4 x + 0.6 y) o

# of electrons

# of electrons

Strong Field and Weak Field

0.8 o 1.2 o

1.6 o 0.6 o

Pairing Energy

1.6 o 0.6 o

Pairing Energy:Coulombic repulsion between electrons in the sameorbital

1.6 o –P (pairing energy) > 0.6 o Strong Field1.6 o –P (pairing energy) < 0.6 o Weak Field

Strong FieldLow spin, 2 unpaired electrons

Weak FieldHigh spin, 4 unpaired electrons

LFSE o and t

0000Zn2+Cu+10

0.410.61Cu2+9

0.821.22Ni2+8

1.230.831.81Co2+7

0.640.442.40Co3+Fe2+6

05052.01Fe3+Mn2+5

0.440.641.62Mn3+Cr2+4

Weak FieldStrong Field

0.831.23V2+Cr3+3

1.220.82V3+2

0.610.41Ti3+1

0000Sc3+Ca2+0

LFSENLFSEN

TetrahedralOctahedralExampledn

Magnetic Measurements

•Paramagnetic:unpaired electrons

•Diamagnetic:all electrons paired

Origins of Magnetism•Self spinning

•Spinning about nucleus

Z+e

Origin of OrbitalAngularMomentum

Spin-only Paramagnetism•Magnetic Moment

= 2{S(S + 1)}1/2 B S = si

= {N(N + 2)}1/2 B

B : Bohr magneton, 9.274 X 10-24 J/T

5.95.925/25Fe3+

4.8 –4.94.9024Mn3+

3.83.873/23Cr3+

2.7 –2.92.8312V3+

1.7 –1.81.731/21Ti3+

Exp.Calc.

/BSNIon

Thermochemical Correlations

Without LFSE

With LFSE

More Stable

Tetrahedral Complexes•Only 4 ligands, all weak field

LFSE o and t

0000Zn2+Cu+10

0.410.61Cu2+9

0.821.22Ni2+8

1.230.831.81Co2+7

0.640.442.40Co3+Fe2+6

05052.01Fe3+Mn2+5

0.440.641.62Mn3+Cr2+4

Weak FieldStrong Field

0.831.23V2+Cr3+3

1.220.82V3+2

0.610.41Ti3+1

0000Sc3+Ca2+0

LFSENLFSEN

TetrahedralOctahedralExampledn

Tetragonal Complexes

t < 1/2 o

Square Planar Complexessp > o

sp ~ 1.3 o

d8 Square Planar stable

Jahn-Teller Effect

•If the ground electronic configuration ofa linear complex is orbitally degenerate,the complex will distort so as to removethe degeneracy and achieve a lowerenergy.

Ligand FieldTheory

a1g

t1u

eg

t2g

MO Energy Level Diagram

CFT

M-L -Bonding

-donor (-base) and -acceptor (-acid)

Effect of -Interaction on o

0 decreasedWeak Field

0 increasedStrong Field

-donor -acceptor

M-L -Bonding•Order of ligand field strength with decreasing o:acceptor > no effect > weak donor > donorstrong field weak field

CO > CN–> PPh3 > phen ~ NO2–> bipy > en >

NH3 ~ py > CH3CN > NCS- > H2O > C2O42–> OH–

> F–> N3- > NO3

–> Cl–> SCN–> S2- > Br–> I–

•Low Oxidation State Metal (electron rich)–Acceptor Ligands

•High Oxidation State Metal (electron poor)–Donor Ligands

PES ofMo(CO)6