The Tolman electronic parameter (TEP) and the metal–metal electronic communication in ditopic NHC complexes

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Whereas the electronic communication between metals in dimetallic organometallic compounds is often assessed through cyclic voltammetric measurements, we have found that the variations in the Tolman electronic parameter (TEP) can also be an alternative and effective way of determining this type of interaction. The TEP values of several (CO)3Ni-NHC-X systems with five different ditopic NHC ligand systems [triazolyldiylidene (A), bis(imidazolylidene) (B), benzobis(imidazolylidene) (C), cyclopenta[f,g]acenaphthylenebis(imidazolylidene) (D) and bis(imidazolidinylidene) (F)] were determined by means of DFT calculations. Based on these values, the electron-withdrawing character of the X metal fragments employed in this study was found to increase in the order IrCp(CO) → RhCl(COD) → Ni(CO)3 → Cr(CO)5 → RhCl(CO)2. We found that the degree of electronic interaction through the ditopic NHC ligands is the strongest in A, followed by B and F, while being weak in B and C. The TEP values and the quantitative analysis of the upper molecular orbitals of A and F and their (CO)3Ni-NHC-Ni(CO)3 complexes strongly suggest that the principal electronic interaction between the metal centres of the M-NHC-M' complexes is of σ-type, via the delocalized HOMO and HOMO - 1 orbitals of the NHC ligands.