Re-examination of the metal carbonyl complex infrared parameter, nu(co), and phosphorus ligand parameters, pKa, Sigma(chi i) and Sigma sigma(ph), in relation to an evaluation of sigma and pi components of M-P bonds

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