Linear semibridging carbonyls—III. Carbonyl and thiocarbonyl ligands as four-electron donors

Abstract

Parameter-free Fenske—Hall molecular orbital calculations predict the electronic structure and bonding of several heterobimetallic complexes containing linear semibridging carbonyl and thiocarbonyl ligands. The results suggest that the linear semibridging ligand acts as a four-electron donor only when the secondary metal centre is an electron-deficient early transition metal atom. When the secondary centre is a late transition metal atom, the linear semibridging ligand acts as a π-acceptor of electron density, even if the late metal has less than 18 electrons. Holes in the d shell of the early metal atoms are more prominent than those in the late metal atoms where the higher nuclear charge pulls the d electrons closer to the nucleus. The role of the semibridging ligand is to fill in the prominent holes in the d shell when they exist by donating electron density from an occupied π orbital. When there are no holes in the d shell, or when the holes are very small, the semibridging ligands accept electron density from the secondary metal centre into a π* orbital.