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), NHCiPr2Me2 (NHCiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon­(IV)-halides, SiCl4, MeSiCl3, Me2SiCl2, and Me3SiCl. 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, NHCMe4 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl4 and Me2SiCl2 adducts even at higher temperatures. As a consequence, NHCiPr2Me2 in donor–acceptor stabilized Si­(II)- and Ge­(II)-dimethyl complexes, NHCiPr2Me2·GeMe2·Fe­(CO)4 and NHCiPr2Me2·SiMe2·Fe­(CO)4, is readily replaced by NHCMe4