Octahedral Molecular Geometry - Splitting of D-orbitals in Octahedral Complexes

Splitting of D-orbitals in Octahedral Complexes

For a "free ion", e.g. gaseous Ni2+ or Mo0, the d-orbitals are equi-energetic, that is they are "degenerate." In an octahedral complex, this degeneracy is lifted. The d_z2 and d_x2−y2, the so-called e_g set, which are aimed directly at the ligands are destabilized. On the other hand, the d_xz, d_xy, and d_yz orbitals, the so-called t_2g set, are not. The labels t_2g and e_g refer to irreducible representations, which describe the symmetry properties of these orbitals. The energy gap separating these two sets is the basis of Crystal Field Theory and the more comprehensive Ligand Field Theory. The loss of degeneracy upon the formation of an octahedral complex from a free ion is called crystal field splitting or ligand field splitting. The energy gap is labeled Δ_o, which varies according to the nature of the ligands. If the symmetry of the complex is lower than octahedral, the e_g and t_2g levels can split further. For example, the t_2g and e_g sets split further in trans-MLa₄Lb₂.

Ligand strength has the following order for these electron donors:

weak: iodine < bromine < fluorine < acetate < oxalate < water < pyridine < cyanide :strong

So called "weak field ligands" give rise to small Δ_o and absorb light at longer wavelengths.