A Computational Study of the Effectiveness of the Frontier Molecular Orbital Formalism in Predicting Conformational Isomerism in (p-RC₆H₄NC)₂W(dppe)₂

Ab initio electronic structure calculations on a series of ligands, p-RC₆H₄NC:, indicate that the energy of the LUMO correlates with the electron-withdrawing/donating capabilities of the substituent group, which determines the relative π-acidity of the ligand. Depending on the nature of the para substituent group on the aryl isocyanide ligand, bis(aryl isocyanide) complexes of tungsten-containing bulky bidentate arylphosphine ligands adopt either cis or trans conformations. The frontier molecular orbital formalism predicts that strong π-acids, which contain electron-withdrawing groups, tend to polarize sufficient charge density away from the metal center to effect the formation of the sterically less favorable but electronically stabilized cis conformer. Density functional theory calculations on similar complexes containing phosphines which do not impose severe steric contraints indicate that the balance between steric and electronic stabilization can be effectively predicted by comparing the relative energies of the ligand LUMOs.