The Lewis basicity of diaminocarbene - A theoretical study of donor-acceptor complexes of C(NH2)(2), NH3 and CO with the Lewis acids EF3, ECl3 (E = B, Al, Ga, In), TiF4 and TiCl4

Quantum chemical calculations at the MP2 level using large valence basis se ts up to TZ+2P quality have been carried out in order to predict the geomet ries and bond energies of the title compounds. The nature of the donor-acce ptor bond has also been investigated. The calculations show clearly that di aminocarbenes are much stronger Lewis bases than amines. The complexation e nergies of C(NH2)(2) have been calculated to be 14-27 kcal/mol higher than those of NH3. The most strongly bonded complex is Cl3Al-C(NH2)(2), which ha s a theoretically predicted Al-C bond energy D-o = 59.1 kcal/mol. In all th e complexes, the strength of the Lewis bases is C(NH2)(2) > NH3 > CO, but t he ordering of Lewis acid strength of EX3 depends on the coordinated Lewis base. TiF4, and TiCl4 have similar Lewis acidities as BF3 but the titanium tetrahalides may bind one or two donor molecules with almost the same bond strength. The investigated donor-acceptor bonds have a high degree of ionic character. The largest covalent contributions are found for the diaminocar bene complexes. The covalent character of the X3E-CO bond increases on goin g from E = boron to the heavier Group 13 elements, while the opposite order is found for the X3E-NH3 and X3E-C(NH2)(2) bonds.