) [Ni(H2O)6]
2+ and [NiCl4]
2-
[NiCl4]
2- shows the more intense ligand-field bands.
Both ions contain Ni2+ (d8
) and the ligand-field transitions are orbitally forbidden (as
Δl = 0 for a ‘d-d’ transition). In the tetrahedral [NiCl4]
2- ion, d-p mixing can occur via
covalent overlap between the metal d and p-orbitals and ligand orbitals. In the
octahedral [Ni(H2O)6]
2+ ion, by symmetry the d-p mixing cannot occur (Laporte
forbidden). However, during some of the molecular vibrations, the centre of inversion
present in the octahedron is lost and the mixing occurs. As the amplitude of vibrations
is very small, the distortion from octahedral symmetry is only ever very small and so
the extent of mixing is low. The bands are thus weak.