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) 5 L {L = PPh 3 , PPh 2 Me, PPhMe 2 } have been determined by NMR spin‐lattice relaxation techniques. Experimental values of axial [2π], compared with those reported for PMe 3 and P(OMe) 3 , reveal that PMe 3 is a slightly better π‐acid than PPh 3 . 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 PMep 3 to PPh 3 . 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 PPh 3 , the better π‐accepting σ* P‐C , as well as the better s‐donating lone‐pair LP(P), in PMe 3 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 PPh 3 < PPh 2 Me < PPhMe 2 < PMe 3 , 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.