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

Abstract

The Tolman electronic parameter (TEP) derived from the A 1symmetrical 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.

Chiral AuI‐ and AuIII‐Isothiourea Complexes: Synthesis, Characterization and Application

“…[28] The electronic effect of the isothiourea ligands was assessed using the To lman electronic parameter (TEP). [29] To determine this parameter,t he synthesis of iridium(I)-isothiourea carbonyl complexes was undertaken.T he reaction of [IrCl(cod)] 2 25 with either (2S,3R)-HyperBTM (2S,3R)-3 or DHPB 1 gave monocoordinated [IrCl{(2S,3R)-HyperBTM}(cod)] (2S,3R)-26 and [IrCl(DHPB)(cod)] 27 as yellow solids in good to excellent yield. [30] In contrast, the reactiono f[ IrCl(cod)] 2 (cod)] 2 25 and free ligand in the presence of the isothiourea catalystw as attempted, only minimal product formation was achieved.…”

Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes

“…In the case of the Tolman parameter, not only the electronic properties of the carbene influence the CO stretching modes, but also steric effects and the coupling of stretching modes. The latter two drawbacks have recently been overcome by calculating the metal–ligand electronic parameter (MLEP) [36]. Separating the influence of the σ-donor and π-acceptor abilities was limited to determining the overall donor character and taking into account the σ-donor character.…”

Dieter Cremer's contribution to the field of theoretical chemistry

“…(4)The local mode analysis revealed that the Tolman electronic parameter (TEP) commonly used to indirectly characterize the metal‐ligand bond does not generally correlate with the intrinsic strength of the metal‐ligand bond as assumed so far. A new electronic parameter was introduced that is directly based on the metal‐ligand local stretching force constant, the metal‐ligand electronic parameter (MLEP), and successfully tested for 181 nickel complexes . This discovery will open new avenues for catalyst design.…”