Re-examination of the Metal Carbonyl Complex Infrared Parameter, v_co, and Phosphorus Ligand Parameters, pKa, Σ_χi and Σσ^ph, in Relation to an Evaluation of σ and Ω Components of M-P Bonds

Abstract

Detailed analyses of the three phosphorus ligand parameters commonly used as gauges of electronic effects in organophosphorus metal chemistry, pKa, Σ_{χ i} and Σσ^ph, are presented in terms of their use as σ measures in M-P bonds. The Tolman ligand parameter, Σ_{χ i}, is shown to contain contributions from σ, Ω and steric factors. Though steric effects can be ignored in the Kabachnik parameter, Σσ^ph, which is a purer σ-parameter than Σ_{χ i} or v_co, 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, v_co 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 v_co as an electronic parameter for M-P bonding.

Key Words:

• M-P bonding
• s-p controversy
• phosphorus ligand parameters