A simplified molecular orbital model for octahedral metal carbonyl compounds
A simple model for the π-electron interaction between metal and CO groups in octahedral metal carbonyl systems has been developed. The basis set employed includes metal d orbitals of π symmetry with respect to the M—CO bond axes, and two orbitals of π symmetry on each CO, corresponding to the vacant π* orbitals of the CO groups. Each CO is assumed to donate 0·5 electrons to the metal via the σ bond. In compounds of the form M (CO)6–nLn, the ligands L are characterized by a certain degree of σ donor ability, and by two acceptor orbitals of π symmetry and variable energy. The model is sufficiently simple to permit exploration of a wide variety of ligand characteristics.
With the aid of assumptions relating the charge distribution to bond orders, relative CO stretching force constants and CO,CO stretching interaction force constants can be calculated as a function of variable ligand characteristics. The results support the adequacy of the simple force field commonly employed in determining relative force constants in substituted octahedral metal carbonyl systems.