raised in energy; and the d xy , d yz and d xz orbitals which are directed between the axes will be lowered in energy relative to the average energy in the spherical crystal field. Thus, the degeneracy of the d orbitals has been removed due to ligand electron-metal electron repulsions in the octahedral complex to yield three orbitals of lower energy, t 2g set and two orbitals of higher energy, e g set. This splitting of the degenerate levels due to the presence of ligands in a definite geometry is termed as crystal field splitting and the energy separation is denoted by D o (the subscript o is for octahedral) (Fig. .
). Thus, the energy of the two e g orbitals will increase by ( / ) D o and that of the three t 2g will decrease by ( / ) D o . The crystal field splitting, D o , depends upon the field produced by the ligand and charge on the metal ion. Some ligands are able to produce strong fields in which case, the splitting will be large whereas others produce weak fields and consequently result in small splitting of d orbitals.
In general, ligands can be arranged in a series in the order of increasing field strength as given below: I – < Br – < SCN – < Cl – < S – < F – < OH – < C O – < H O < NCS < edta – < NH < en < CN – < CO Such a series is termed as spectrochemical series . It is an experimentally determined series based on the absorption of light by complexes with different ligands. Let us assign electrons in the d orbitals of metal ion in octahedral coordination entities. Obviously, the single d electron occupies one of the lower energy t 2g orbitals.
In d and d coordination entities, the d electrons occupy the t 2g orbitals singly in accordance with the Hund’s rule. For d ions, two possible patterns of electron distribution arise: (i) the fourth electron