Synthesis and Structural Studies of Nickel(0) Tetracarbene Complexes with the Introduction of a New Four-Coordinate Geometric Index, τδ

The synthesis and characterization of two homoleptic chelating nickel(0) tetracarbene complexes are reported. These are the first group 10 M(0) (M = Ni, Pd, Pt) tetracarbene complexes. These species have geometries intermediate between C2v sawhorse and tetrahedral and show high UV–vis absorption in the 350–600 nm range, with extinction coefficients (ϵ) between 5600 and 9400 M–1 cm–1. Density functional theory analysis indicates that this high absorptivity is due to metal-to-ligand charge transfer. In order to better describe the unusual geometries encountered in these complexes, an adjustment to the popular τ4 index for four-coordinate geometries is introduced in order to better delineate between sawhorse and distorted tetrahedral geometries.