The rich structural chemistry displayed by the carbon monoxide as a ligand to metal complexes

Abstract

The diatomic CO molecule is a very important ligand in organometallic chemistry. The bond between the carbonyl and a metal is moderately strong and consists of a sigma bond, formed by donation of electron density to the metal from the carbonyl’s highest occupied molecular orbital (HOMO, the 5σ), and π bonds, formed by donation of electron density from the metal to the carbonyl’s lowest unoccupied molecular orbital (LUMO, the 2π). The carbonyl may also serve as a bridging ligand connecting two or more metal atoms. Depending on the relative orientation between the carbonyl and metals, one may classify a bridging carbonyl as symmetric bridging, bent semibridging, linear semibridging, face bridging, and bridging isocarbonyls. The rich structural chemistry displayed arises from a complex interplay between the metal’s electronic structure and the carbonyl’s 5σ and 2π. In addition, the carbonyl’s occupied 1π and 4σ orbitals may in certain cases donate electrons when it binds to electron-deficient metals, further complicating the electronic structure. Such complexity in the carbonyl-metal interaction raises challenges to the simple applications of Lewis bonding ideas and electron counting rules. Therefore, theoretical analyses have been applied, largely in a case-by-case pattern, to investigate the rationales behind the CO’s rich structural chemistry.