Natural Bond Orbital Rationalizations of NMR Observations for Metal‐Ligand Bonding (II): Rehybridization of Phosphorus Arising from Coordination of Methyl‐Phenyl‐Phosphines

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 PMep₃ 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 s‐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 d_π → σ*_P‐Y back‐donation.