We report the reversible coordination of the N-heterocyclic carbene (NHC), NHCiPr2Me2 (NHCiPr2Me2 = 1,3-diisopropy1-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiC14, MeSiC13, Me2SiC12, 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 a-donating N-heterocyclic carbene, NHCMe4 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiC14 and Me2SiC12 adducts even at higher temperatures. As a consequence, NHCPrzNi" in donor acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, (NHCGeMe2)-Ge-iPr2Me2-Fe(CI())4 and NHCiPr2Me2.SiMe2 Fe(CO)(4) is readily replaced by NHCMe4.