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

Abstract

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'_, τ_δ, τ_tet-sq, etc. are described along with their exceptions.

Keywords:

• Addison parameter
• distorted geometry
• geometry index
• square pyramidal vs. trigonal bipyramidal
• tetrahedral vs. square planar

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors gratefully acknowledge the Science and Engineering Research Board (SERB), New Delhi, India for a start-up research grant (File Number: SRG/2022/001173) for financial assistance.