ligand field theory - home - chemistry

Metal dx2-y2

NH3

Right symmetry for interaction

Metal dxy

Wrong symmetry for interaction

x

y

NH3x

So, (3dz2, & 3dx2-y2) & 2 lone pair orbitals

interact to form sigma bonds

dxy, dxz, dyz do NOT

y

Octahedral Case: Ligands along x, y, z axes

σ-bonding: Molecular Orbital Theory (LCAO-MO)

applied to Coordination Compounds: Ligand Field Theory

Metal Atom Orbitals (ligands along x, y, z axes)

(3dz2, & 3dx2-y2)

Ligands (6 lone pair orbitals)

Ligand Field Theory

SALC

(sigma)6 0 0 2 2 0 0 0 4 2 = A1g + Eg + T1u

Reducible Representation Decompose into three

Irreducible Representations

Ligand Field Theory: Oh Complexes

FIGURE 20.16

SALCs Resource section 5

Symmetry of d orbitals From Oh character table

Crystal Field Splitting of

Tetrahedral Complexes

e = low energy

t2 = high energy (closer to corners)

note: NO “g” subscripts for d orbital symmetry in tetrahedral geometry(the Td point group does not have the inversion symmetry)

Ligand Field Theory: Td Complexes

SALCs from Resource section 5

Metal ML4 4 Ligands

3d

4s

4pT2

A1

E+T2

s

A1+T2

a1

a1

t2

t2

t2

Metal ML4 4 Ligands

3d

4s

4pT2

A1

E+T2

s

A1+T2

a1

a1

t2

t2

t2

e

DT

Crystal Field Splitting

Tetrahedral Complexes

No low-spin tetrahedral complexes!

Dt= -4/9 Do

Extent of splitting from p - bonding: Weak and Strong Field ligands

Consider Cl- (weak), NH3 (intermediate) and CO (strong)

CASE 1: NO p-INTERACTION = σ-DONOR (i.e. NH3)

Metal dx2-y2

NH3x

y

Extent of splitting from p - bonding: Weak and Strong Field ligands

Consider Cl- (weak), NH3 (intermediate) and CO (strong)

Cl M

- bonding as before

Now p - bonding between p & dxy, dxz, dyz

σ - bonding as before

Now p - bonding between CO p * & dxy, dxz, dyz

No p - bonding with CO p

M

CASE 2: p-DONOR (i.e. Cl-)

CASE 3: p -ACCEPTOR (i.e. CO)

CASE 1: NO p-INTERACTION = σ-DONOR (i.e. NH3)

N C

SALCs from Resource section 5

dxy, dxz, dyz

d* = eg

= t2g

σ metal-ligand molecular orbitals

(all filled, mostly ligand character)

6 ligand donor orbitals

(sigma symmetry)

Metal LigandMolecule

Metal-Ligand Bonding: Sigma-DONOR Ligands, NO pi-bonding

Do

σ* metal-ligand molecular orbitals

(all empty, mostly metal character)

d

s

p

A1g + Eg + T1u

Eg + T2g

A1g

T1u

dxy, dxz, dyz

d* = eg

Ligand donor orbitals

(pi symmetry)

always lower energy

than metal orbitals

Metal LigandMolecule

Metal-Ligand Bonding: Sigma-Donor, Pi-Donor Ligands

Do

d

s

p

Pi-Donor Ligands

DECREASE Do

= Weak Field Ligands

p*-antibonding orbitals

p-bonding orbitals

Eg + T2g

A1g

T1u

sigma-donororbital

pi-acceptingorbital(only one of the two)

Classical pi-acceptor:Carbonyl (CO)

dxy, dxz, dyz

d* = eg

Ligand acceptor orbitals

(pi symmetry)

always higher energy

than metal orbitals

Metal LigandMolecule

Metal-Ligand Bonding: Sigma-Donor, Pi-Acceptor Ligands

Dod

s

p

Pi-Acceptor Ligands

INCREASE Do

= Strong-Field Ligands

p*-antibonding orbitals

p-bonding orbitals

Eg + T2g

A1g

T1u

Pi-Backbonding with Pi-Acceptor Ligands

Two contribution to the overall bonding:

The backbonding effect stabilizes the complex because the overall charge transfer can be adjusted to fit both the ligand and metal “needs”: if the metal would like to have more or less electrons, it can adjust the amount of backdonation to the ligand.

Backdonation tends to favor low-oxidation state metals, such as Ti(0) or Cr(0) for instance.

Strong field

Ligands

Weak field

Ligands

The spectrochemical series

Predicting the Crystal Field Splittings (p. 670)

xz yz xy

z2 x2-y2

Do

xz yz xy

z2 x2-y2

Do

Do = hn

I- < Br- < [NCS]- < Cl- < F- < [OH]- < [ox]2- ~ H2O< [NCS]- < NH3 < en < bpy < phen < [CN]- ~ CO