On the Bonding Situation in Stannocene and Plumbocene N-Heterocyclic Carbene Complexes

A detailed experimental and computational study of a series of stannocene and plumbocene N-heterocyclic carbene complexes is presented. This unique class of group 14 Lewis acid base adducts was obtained from reactions of the corresponding metallocenes and N-heterocyclic carbenes (NHC), and were structurally characterized by single crystal X-ray diffraction. The obtained structures show a perpendicular pose of the NHC with respect to the metallocene, hence precluding the maximal interaction between the moieties. The nature of the Sn-CNHC and Pb-CNHC bonds have been investigated by applying Energy Decomposition Analysis (EDA-NOCV). For the sake of comparison, known stannocene and plumbocene Lewis base complexes have been included in the series. The attractive chemical bonding interactions are around 50% electrostatic, 30% covalent and 20% dispersion. Indeed, dispersion interactions play a determining role the bigger the substituents become. The covalent interactions derive from the donation of the carbene ligand into the empty p orbital of the metallocene.