Molecular electrostatic potential approach to determining the steric effect of phosphine ligands in organometallic chemistry

A two-layer ONIOM(B3LYP/6-31G(d, p): UFF) quantum mechanics (QM) -molecular mechanics ( MM) optimization of PR3 ligands, where the QM layer is always constructed as PH3, followed by molecular electrostatic potential ( MESP) analysis of the QM layer is suggested as a simple and effective method for evaluating the steric effect of PR3 ligands. The subtle variations in the electron distribution that arise as a result of the steric bulkiness as well as the conformational changes in the substituent groups is well reflected in the value of the MESP minimum (V-min) located in the QM region. In general, a sterically bulky group has always shown a more negative V-min than a sterically less bulky group. The difference between the V-min value of free PH3 and the V-min value at the QM region of PR3 is used as a measure of the steric effect of the PR3 ligand. This value, designated as MESPsteric, showed a good linear correlation with the cone angle values as well as the average of the intervalence HPH angles found in the QM layer. Further, the difference between the V-min value at the QM region of PR3 ( an indicator of the steric effect) and the V-min value of the completely optimized PR3 ligand ( an indicator of the combined effect of steric and electronic effects) showed a good linear correlation with the Hammett Sigma sigma(p) constants, which further confirmed the present approach to understanding the steric effect separately from the electronic effect.