Crystal Field Theory i Separate metal and ligands

  • Slides: 12
Download presentation
Crystal Field Theory i) Separate metal and ligands have high energy ii) Coordinated Metal

Crystal 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 = q 1 q 2 ( q 1 q 2 – point charges, distance between charges) r 2 vi) Destabilization due to ligand -d electron repulsion

d - orbitals

d - orbitals

Ligand-Metal Interaction • Basic Assumption in CFT: • Electrostatic interaction between ligand metal d-orbitals

Ligand-Metal Interaction • Basic Assumption in CFT: • Electrostatic interaction between ligand metal d-orbitals align along the octahedral axis will be affected the most. More directly the ligand attacks the metal orbital, the higher the energy of the d-orbital. In an octahedral field the degeneracy of the five d-orbitals is lifted

Octahedral field

Octahedral field

Splitting of d-Orbitals Octahedral Field • Ligands approach metal on the axis d-orbitals pointing

Splitting of d-Orbitals Octahedral Field • Ligands approach metal on the axis d-orbitals pointing directly at axis are affected most by electrostatic interaction d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction

Splitting in Octahedral field Stage II Hypothetical Stage III

Splitting in Octahedral field Stage II Hypothetical Stage III

Splitting in Octahedral field Stage I – The free metal ion. Degenerate d- orbitals.

Splitting in Octahedral field 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 x 2 – y 2 and dz 2 ) 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 t 2 g orbital energy equal to 4 Dq is released and every electron entering eg orbital raises the energy equal to 6 Dq.

Splitting in Tetrahedral field z - axis y - axis X - axis

Splitting in Tetrahedral field z - axis y - axis X - axis

Splitting in Tetrahedral field • Free metal ion • Spherical field • Degenerate d

Splitting in Tetrahedral field • Free metal ion • Spherical field • Degenerate d – orbitals • Higher energy • Free metal ion • Degenerate d - orbitals splitting of d – orbitals CFS = Δt

Splitting in Tetrahedral field Stage I – The free metal ion. Degenerate d- orbitals.

Splitting in Tetrahedral 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 a tetrahedral field. 2. Ligands approach in between the axis. 3. The t 2 g 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 Square Planar field d x 2 – y 2 Δsp d x

Splitting in Square Planar field d x 2 – y 2 Δsp d x 2 – y 2 d z 2 d xy d z 2 d xz d yz State 1 State 2 State 3 State 4 State 5

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

Splitting 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 dz 2 are lowered in energy. Stage V - Orbitals oriented along the x – axis are removed hence dxz, dyz and dz 2 are further lowered in energy and difference in energy between dxy and dx 2 –y 2 give the crystall field splitting energy.