Solid-state spectral perturbations in square-planar complexes

A brief overview of color changes of square-planar complexes upon crystallization is presented along with selected spectra and theoretical spectral modeling results. In general, brilliant colored crystals of several square-planar complexes develop from colorless solution. The systems include Magnus’ green salt, tetracyanometallates, and bis(dimethylglyoximato)nickel(II). It is generally accepted that the transition responsible for a spectral red shift and increase in intensity is from an uppermost filled orbital with a large metal d z 2 component to a low-lying empty metal-based pz orbital. Since these orbitals are largely oriented out of plane, they interact most strongly when the planes are aggregated. Experimental data from selected single-crystal polarized specular reflectance spectra and ZINDO calculations on several systems are reviewed.