Theoretical Studies of Inorganic Compounds, IX The Lewis Basicity of Diaminocarbene-A Theoretical Study of Donor-Acceptor complexes of C(NH_2)_2, NH_3 and CO with the Lewis Acids EF_3, ECl_3 (E = B, Al, Ca, In), TiF_4 and TiCl_4

摘要

Quantum chemical calculations at the MP2 level using large valence basis sets up to TZ+2P quality have been carried out in order to predict the geometries and bond energies of the title compounds. The nature of the donor-acceptor bond has also been investigated. The calculations show clearly that diaminocarbenes are much stronger Lewis bases than amines. The complexation energies of C(NH_2)_2 have been calculated to be 14-27 kcal/mol higher than those of NH_3. The most strongly bonded complex is Cl_3Al-C(NH_2)_2, which has a theoretically predicted Al-C bond energy D_o = 59.1 kcal/mol. In all the complexes, the strength of the Lewis bases is C(NH_2)_2 > NH_3 > CO, but the ordering of Lewis acid strength of EX_3 depends on the coordinated Lewis base. TiF_4 and TiCl_4 have similar Lewis acidities as BF_3, 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 diaminocarbene complexes. The covalent character of the X_3E-CO bond increases on going from E = boron to the heavier Group 13 elements, while the opposite order is found for the X_3E-NH_3 and X_3E-C(NH_2)_2 bonds.