A computational study of the effectiveness of the frontier molecular orbital formalism in predicting conformational isomerism in (p-RC6H4NC)(2)W(dppe)(2)

Ab initio electronic structure calculations on a series of ligands, p-RC6H4 NC:, indicate that the energy of the LUMO correlates with the electron-with drawing/donating capabilities of the substituent group, which determines th e relative pi -acidity of the ligand. Depending on the nature of the para s ubstituent group on the aryl isocyanide ligand, bis(aryl isocyanide) comple xes of tungsten-containing bulky bidentate arylphosphine ligands adopt eith er cis or trans conformations. The frontier molecular orbital formalism pre dicts that strong pi -acids, which contain electron-withdrawing groups, ten d to polarize sufficient charge density away from the metal center to effec t the formation of the sterically less favorable but electronically stabili zed cis conformer. Density functional theory calculations on similar comple xes containing phosphines which do not impose severe steric contraints indi cate that the balance between steric and electronic stabilization can be ef fectively predicted by comparing the relative energies of the ligand LUMOs.