Bonding in metal-carbonyls:: A comparison with experiment and calculations on adsorbed CO

The general aspects of CO bonding in carbonyls and on metal surfaces are discussed in terms of a molecular orbital description, based on calculations and direct experimental measurements. In particular, the excellent agreement between experiment and theory for CO adsorbed on Ni(100) is the starting point to derive in the same theoretical approach the orbital-based electronic structure of the carbonyls Ni(CO)4, Fe(CO)5, Cr(CO)(6) and CO coordinated to the heme group. A frontier orbital interaction scheme involving only the 2 pi* and the 5 sigma orbitals is shown to be inadequate for the description of the electronic structure; all orbitals change due to the interaction. Instead, we demonstrate how the formation of an allylic configuration in the pi-system, involving both the 1 pi and 2 pi* orbitals, gives a more complete description of the pi electronic structure. The rearrangement of the sigma orbitals involves mainly mixing of occupied CO and metal orbitals resulting in Pauli repulsion. The latter is minimized by polarization of charge away from the CO-Me bond. The bonding picture for CO in organometallic systems is very similar to the surface adsorption case; the main idea of repulsive sigma and bonding pi interactions seems generally valid when CO interacts with metals. (c) 2005 Elsevier B.V. All rights reserved.