Computational Analysis of n→π* Back-Bonding in Metallylene-Isocyanide Complexes R2MCNR′ (M = Si, Ge, Sn; R = tBu, Ph; R′ = Me, tBu, Ph)

Abstract

A detailed computational investigation of orbital interactions in metal–carbon bonds of metallylene–isocyanide adducts of the type R2MCNR′ (M = Si, Ge, Sn; R, R′ = alkyl, aryl) was performed using density functional theory and different methods based on energy decomposition analysis. Similar analyses have not been carried out before for metal complexes of isocyanides, even though the related carbonyl complexes have been under intense investigations throughout the years. The results of our work reveal that the relative importance of π-type back-bonding interactions in these systems increases in the sequence Sn < Ge ≪ Si, and in contrast to some earlier assumptions, the π-component cannot be neglected for any of the systems investigated. While the fundamental ligand properties of isocyanides are very similar to those of carbonyl, there are significant variations in the magnitudes of different effects observed. Most notably, on coordination to metals, both ligands can display positive or negative shifts in their characteristic stretching frequencies. However, because isocyanides are stronger σ donors, the metal-induced changes in the CN bonding framework are greater than those observed for carbonyl. Consequently, isocyanides readily exhibit positive CN stretching frequency shifts even in complexes where they function as π-acceptors, and the sign of these shifts is alone a poor indicator of the nature of the metal–carbon interaction. On the other hand, the relative π-character of the metal–carbon bond in metallylene–isocyanide adducts, as judged by the natural orbitals of chemical valence as well as by partitions of the orbital interaction energy, was shown to have a linear correlation with the shift in CN stretching frequency upon complex formation. The details of this correlation show that π-back-donation contributions to total orbital interaction energy need to exceed 100 kJ mol–1 in order for the shift in the CN stretching frequency of metallylene–isocyanide adducts to be negative.