A four-parameter system for rationalising the electronic properties of transition metal-radical ligand complexes

A system of four principal parameters is reported that provides a unified description of the electronic and chemical properties of radical-ligand coordination compounds. This type of parametrisation applies to compositionally different types of radical-ligands, and the principal parameters rank in the following order: (a) coordination mode (metal-ligand orbital alignment) > (b) metal linkage > (c) ligand charge > (d) geometric strain (on orbital overlap). A series of group-10 metal complexes of an open-shell thiolate-arene-thiolate ligand suits to differentiate between three of the four effects in a clear-cut fashion, which allowed sorting these into a semi-quantitative order for the first time. Combined experimental and TD-DFT data aided in distinguishing structural effects from metal specific contributions such as relativistic effects. The applicability of spectroscopic and structure properties to serve as characteristic markers for comparison is discussed with regard to the large body of planar radical-ligand structures. A heuristic four-parameter scheme captures and predicts the electronic properties of radical-ligand transition metal compounds, overcoming ligand specific descriptions.