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
was 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 maximum interaction
between the moieties. The nature of the Sn–CNHC and
Pb–CNHC bonds has been investigated by applying
natural bond orbital (NBO) analysis and 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 larger the substituents become. The covalent interactions
derive from the donation of the carbene ligand into the empty p orbital
of the metallocene.