Question

Briefly discuss the crystal fieid splitting in octahedral complexes.

Answer 1

As a result of repulsive forces between the ‘d’ electrons of the metal ion and that of ligands (negative or polar neutral), the degeneracy of the ‘d’ orbitals is lost. Since the two lobes of two e_g{d_x²-y², d_z²) orbitals lie in the path of approaching ligands these orbitals experience greater forces of repulsion than those of t_2g (d_xy, d_yz and d_xz) orbitals is decreased.

Thus, an energy difference exists between two sets of orbitals. This energy difference is called crystal field splitting energy and is represented by Δ_o. (the subscript ‘o’ stands for octahedral). It measures the crystal field strength of the ligands. The crystal field splitting occurs in such a way that the average energy of the ‘d’ orbitals do not change.

(a) Five degenerate ‘d’ orbitals of the free metal ion.

(b) Hypothetical degenerate ‘GP orbitals at higher energy levels under spherically symmetrical ligand field.

(c) Splitting of ‘d’ orbitals under the influence of ligands.

Thus, three orbitals lie at an energy i.e., 2/5 Δ_o below the average ‘d’ orbital energy and two ‘d’ orbitals lie at an energy 3/5 Δ_o above the average energy. The energy gap between t_2g and e_g sets also denoted by 10Dq.

The energy of t_2g orbitals is 4Dq less than that of hypothetical degenerate ‘d’ orbitals and that of e_g orbitals is 6Dq above that of hypothetical degenerate ‘d’ orbitals: Thus, t_2g set loses energy equal to 0.4 Δ_o or 4 Dq white e_g sets gain energy equal to 0.6 Δ_o or 6 Dq.