Natural Bond Orbital Rationalizations of NMR Observations for Metal‐Ligand Bonding (II): Rehybridization of Phosphorus Arising from Coordination of Methyl‐Phenyl‐Phosphines |
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Authors: | Wei‐Lin Su Hsiu‐Ping Huang Wei‐Ting Chen Wen‐Yi Hsu Hsi‐Yen Chang Szu‐Yu Ho Shao‐Pin Wang Shin‐Guang Shyu |
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Affiliation: | 1. Department of Chemistry, National Cheng Kung University, No. 1, University Road, Tainan City 701, Taiwan, R.O.C.;2. Institue of Chemistry, Academia Sinica, Taipei 115, Taiwan, R.O.C. |
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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 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 dπ → σ*P‐Y back‐donation. |
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Keywords: | NMR spin‐lattice relaxation times Natural bond orbital (NBO) analysis Methyl‐phenyl‐phosphines Rehybridization of phosphorus lone‐pair |
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