Silavinylidene stabilized by an N-heterocyclic carbene: a theoretically predicted stable molecule.

Quantum chemical calculations show that the N-heterocyclic carbene (NHC)-stabilized silavinylidene, NHC(tBu)→C=SiR2 is a strongly bonded complex, which has a linear arrangement of the donor and acceptor moieties. The molecule is the energetically lowest lying isomer of the NHC-stabilized R2CSi isomers and it is stable towards dimerization when R is a bulky substituent. The silavinylidene complex is the only species of the silylidene homologues NHC(tBu)→E=E'R2 (E, E' = C-Pb) which possesses a linear arrangement. The unusual bonding situation is explained in terms of donor-acceptor interactions between NHC(tBu) as σ donor and C=SiR2 in the doubly excited singlet state 3a1⇒2b2 which leads to a significantly shorter C-SiR2 bond compared with free C=SiR2 .