Re-examination of the Metal Carbonyl Complex Infrared Parameter, vco, and Phosphorus Ligand Parameters, pKa, Σχiand Σσph, in Relation to an Evaluation of σ and Ω Components of M-P Bonds

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Abstract
Detailed analyses of the three phosphorus ligand parameters commonly used as gauges of electronic effects in organophosphorus metal chemistry, pKa, Σχ i and Σσph, are presented in terms of their use as σ measures in M-P bonds. The Tolman ligand parameter, Σχ i, is shown to contain contributions from σ, Ω and steric factors. Though steric effects can be ignored in the Kabachnik parameter, Σσph, which is a purer σ-parameter than Σχ i or vco, it suffers from its derivation as a Hammett parameter. It is concluded that pKa is the best choice as a σ-only parameter for phosphorus ligands. Furthermore, vco is shown to be inappropriate as a σ-only or Ω-only parameter, and the force constant k is deemed to be no better than vco as an electronic parameter for M-P bonding.