Equilibrium Coordination of NHCs to Si(IV) Species and Donor Exchange in Donor-Acceptor Stabilized Si(II) and Ge(II) Compounds.

We report the reversible coordination of the N-heterocyclic carbene (NHC), NHC (NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiCl, MeSiCl, MeSiCl, and MeSiCl. Predicted as well as experimentally determined thermodynamic parameters of these equilibria confirm that the complexation constant increases with the Lewis acidity of the silicon halides. In contrast, the more σ-donating N-heterocyclic carbene, NHC (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl and MeSiCl adducts even at higher temperatures. As a consequence, NHC in donor-acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, NHC ·GeMe·Fe(CO) and NHC ·SiMe·Fe(CO), is readily replaced by NHC.