COMPARISON OF ISOELECTRONIC ALUMINUM NITROGEN AND SILICON CARBON DOUBLE-BONDS USING VALENCE BOND METHODS

In the reaction between trimethylaluminum and ammonia to form aluminum nitride, (CH3)2AlNH2 is a postulated intermediate. Results of ab initio geometry optimization calculations for this species as well as H2AlNH2 and isoelectronic H2SiCH2 are presented. Each of these has a planar equilibrium skeleton with C2v symmetry. Geometry optimizations were carried out by using generalized valence bond (GVB) wave functions. Al = N bond distances of 1.78 and 1.80 angstrom are predicted for the dihydro- and dimethyl-aluminum amides, respectively, which are slightly longer than the optimized Si = C bond distance of 1.74 angstrom in H2SiCH2. Al = N bond distances in these compounds are found to agree with a phenomenological correlation established by Haaland, which relates the ratio of covalent to dative character of such bonds to the observed bond distances. We compare the bonding in Al = N and Si = C molecules by analyzing the nature of the GVB orbitals describing the bonds and comparing their predicted dipole moments.