Geometry indices for four- and five-coordinate metal complexes: a concise review

This concise review describes the geometry indices for four- and five-coordinate metal complexes to understand the concepts for quantifying deviations from ideal geometries. While discussing structures of metal complexes, the term 'distorted' is often used to describe geometries. However, this is a qualitative measurement of deviation. It only points out that a geometry differs from an ideal state. The geometry index or structural parameter (τ) quantifies the actual deviation, describing the extent of the deviation and from which geometry it deviates. This review summarizes the progress of the various equations that have been developed over time and provides some examples to verify the equations. Emphasis is mainly given to four- and five-coordinate transition metal complexes and the geometries associated with them. The structural parameters τ4 (for four-coordinate geometry) and τ5 (for five-coordinate geometry) and their modified versions such as τ4', τδ, τtet-sq, etc. are described along with their exceptions.