Natural bond orbital rationalizations of NMR observations for metal-ligand bonding (II): Rehybridization of phosphorus arising from coordination of methyl-phenyl-phosphines

Abstract

Carbonyls' 2π orbital populations, [2π], in W(CO)5L {L = PPh3, PPh2Me, PPhMe2} have been determined by NMR spin-lattice relaxation techniques. Experimental values of axial [2π], compared with those reported for PMe3 and P(OMe)3, reveal that PMe3 is a slightly better π-acid than PPh3. Through space interactions between carbonyl and phenyl groups are insignificant since values of [2π] do not vary significantly in the series of phosphines, going from PMe3 to PPh3. Natural bond orbital (NBO) studies indicate that π-accepting capabilities for these phosphines are primarily governed by the nature of P-C anti-bonding, σ* P-C. Compared with PPh3, the better π-accepting σ* P-C, as well as the better σ-donating lone-pair LP(P), in PMe3 can both be explained by the higher extents of rehybridization of the coordinated phosphorus atom. Based on this rehybridization argument, the NBO predicted order of increasing π-acidic strengths PPh3 < PPh2Me < PPhMe2 < PMe3, which cannot be clearly distinguished by NMR experiments, is ascribed to the same NBO trend of σ-donating capabilities in a synergistic manner. Effects of coordination on P-Y (Y = C, O, F) bonding strengths in phosphines (or phosphites) are depending on two conflicting effects: rehybridization of LP(P) and the hyperconjugative-like dπ → σ*P-Y back-donation.