Crystal Field TheoryCrystal Field Theory

i) Separate metal and ligands have high energy

ii) Coordinated Metal - ligand get stabilized

iii) Metal and Ligands act as point charges.

iv) Metal and ligands bond due to strong electrostatic forces

of attraction.

v) F = q1 q2 (q1 q2 – point charges, distance between charges)

r2

vi)Destabilization due to ligand -d electron repulsion

d - orbitalsd - orbitals

Ligand-Metal InteractionLigand-Metal Interaction

• Basic Assumption in CFT:• Electrostatic interaction between ligand and metal

d-orbitals align along the octahedral d-orbitals align along the octahedral axis will be affected the most.axis will be affected the most.

More directly the ligand attacks the More directly the ligand attacks the metal orbital, the higher the the metal orbital, the higher the the energy of the d-orbital.energy of the d-orbital.

In an octahedral field the In an octahedral field the degeneracy of the five d-orbitals is degeneracy of the five d-orbitals is liftedlifted

Octahedral field

Splitting of d-Orbitals Splitting of d-Orbitals Octahedral Field

•Ligands approach metal on the axis

d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction

d-orbitals pointing directly at axis are affected most by electrostatic interaction

Splitting in Octahedral fieldSplitting in Octahedral field

Stage I Stage II Stage III

Hypothetical

Stage I – The free metal ion. Degenerate d- orbitals.

Stage II – Hypothetical stage-

1. The six ligands create a symmetrical uniform field.

2. All the d orbital experience equal repulsion and their energy get raised equally.

3. Degenerate d- orbitals.

Stage III –

1.Ligands create an octahedral field and approach on the axis.

2.The eg , d orbitals oriented along the axis (d x2 – y2 and dz2 ) experience greater

repulsion and are repelled more.

3.Degeneracy is lost.

4.Splitting of d orbitals take place, Δo = 10 Dq

5.For every electron entering t2g orbital energy equal to 4 Dq is released and

every electron entering eg orbital raises the energy equal to 6 Dq.

Splitting in Octahedral fieldSplitting in Octahedral field

Splitting in Tetrahedral fieldSplitting in Tetrahedral field

X - axis

y - axis

z - axis

Splitting in Tetrahedral fieldSplitting in Tetrahedral field

•Free metal ion•Degenerate d - orbitals

•Free metal ion•Spherical field•Degenerate d – orbitals•Higher energy

splitting of d – orbitalsCFS = Δt

Stage I – The free metal ion. Degenerate d- orbitals.

Stage II –

Hypothetical stage –

1. The four ligands create a symmetrical uniform field.

2. All the d orbital experience equal repulsion and their energy get raised equally.

3. Degenerate d- orbitals.

Stage III –

1.Ligands create a tetrahedral field.

2.Ligands approach in between the axis.

3.The t2g d orbitals oriented in between the axis (dxy, dyz , dxz) experience greater

repulsion and are repelled more.

4.Degeneracy is lost.

5.Splitting of d orbitals take place.

Splitting in Tetrahedral fieldSplitting in Tetrahedral field

d x2 – y2

d z2

d xy

d xz d yz

d x2 – y2

d z2

d xy

d yz d xz

Δsp

State 1 State 2 State 3 State 4 State 5

Splitting in Square Planar fieldSplitting in Square Planar field

Splitting in Square Planar fieldSplitting in Square Planar field Stage I – The free metal ion. Degenerate d- orbitals.

Stage II – Hypothetical stage –

1. The four ligands create a symmetrical uniform field.

2. All the d orbital experience equal repulsion and their energy get raised equally.

3. Degenerate d- orbitals.

Stage III –

1.Ligands create an octahedral field. Ligands approach in between the axis.

2. Splitting of d orbitals take place.

Stage IV – Orbitals oriented along the x – axis are moved away from the central metal,

hence dxz, dyz and dz2 are lowered in energy.

Stage V - Orbitals oriented along the x – axis are removed hence dxz, dyz and dz2 are further

lowered in energy and difference in energy between dxy and dx2 –y2 give the crystall field

splitting energy.