Quantifying the electronic effect of substituted phosphine ligands via molecular electrostatic potential.

Values of the molecular electrostatic potential minimum (V(min)) corresponding to the lone pair region of several substituted phosphine ligands (PR(3)) have been determined at the DFT level. The V(min) value is proposed as a quantitative measure of the electronic effect of the PR(3) ligands. Good linear correlation between V(min) and Tolman electronic parameter of PR(3) has been obtained. V(min) is also proportional to the pK(a) values of the conjugate acids of PR(3), viz., [PR(3)H](+). Further, the DeltaE values of the reaction Ni(CO)(3) + PR(3) --> Ni(CO)(3)PR(3) and ScH(3) + PR(3) --> ScH(3)PR(3) are also linearly proportional to the V(min) values. However, if there is a strong metal to phosphorus pi-back-bonding, the DeltaE and V(min) do not fit to a line. It is also found that the standard reduction potential as well as the enthalpy change corresponding to the electrochemical couple eta-Cp(CO)(PR(3))(COMe)Fe(+)/eta-Cp(CO)(PR(3))(COMe)Fe(0) is linearly proportional to the V(min) values of PR(3). These correlations suggest that V(min) is a quantitative measure of the sigma-donating ability of the phosphine. It is hoped that, in phosphine-metal coordination chemistry, the V(min) based electronic parameter could be more advantageous than nu-CO and pK(a) based electronic parameters as it solely represents the inherent electronic property of the ligand.