Direct Measure of Metal–Ligand Bonding Replacing the Tolman Electronic Parameter

The Tolman electronic parameter (TEP) derived from the A 1-symmetrical CO stretching frequency of nickel-tricarbonyl complexes L–Ni­(CO)3 with varying ligands L is misleading as (i) it is not based on a mode decoupled CO stretching frequency and (ii) a generally applicable and quantitatively correct or at least qualitatively reasonable relationship between the TEP and the metal–ligand bond strength does not exist. This is shown for a set of 181 nickel-tricarbonyl complexes using both experimental and calculated TEP values. Even the use of mode–mode decoupled CO stretching frequencies (L­(ocal)­TEPs) does not lead to a reliable description of the metal–ligand bond strength. This is obtained by introducing a new electronic parameter that is directly based on the metal–ligand local stretching force constant. For the test set of 181 nickel complexes, a direct metal–ligand electronic parameter (MLEP) in the form of a bond strength order is derived, which reveals that phosphines and related ligands (amines, arsines, stibines, bismuthines) are bonded to Ni both by σ-donation and π-back-donation. The strongest Ni–L bonds are identified for carbenes and cationic ligands. The new MLEP quantitatively assesses electronic and steric factors.