Ab initio calculations of closo-alane and gallane A(6)H(6)(2-) and A(12)H(12)(2-) anions and Al12H12L salts with L = Li+, Na+, Be2+, Mg2+, Al+, Al3+ and H+ cations coordinated inside and outside the icosahedral Al-12 cluster

Ab initio calculations of geometrical, vibrational, magnetic, and energetic parameters of closo-alane and gallane Al6H62-, Ga6H62-, Al12H122-, and Ga1 2H122- dianions and Al6H6L and Al12H12L salts with light alkali-metal and a lkaline-earth cations or proton (L = Li+, Na+, Be2+, Mg2+, Al+, Al3+, H+) w ere performed at the Hartree-Fock (HF) level of theory and within the frame work of the density functional theory (B3LYP) with 6-31G" and 6-311+G"" bas is sets. It was found that the A112H12 a"d Gal2H12 structures with close R( AlAl) and R(GaGa) distances. The most favorable structures of Al12H12L (and , probably, of Ga12H12L) salts are tridentate isomers C-3v(t) with the L ca tion coordinated to the center of one of the 20 equivalent faces (Al-3). Th e potential-energy barrier (bidentate structure C-2v(b)) between the C-3v(t ) structures is small and, similarly to borane analogues, alane salts shoul d be nonrigid from the standpoint of migration of the outer cation around t he icosahedral [Al-12] cluster from one face to another over the adjacent e dge. As in the case of closo-boranes, the symmetrical Ih structure with the cation at the center of the Al-12 icosahedron is untypical for Al12H12L mo lecules (lies very high and possessed six or more imaginary frequencies) wi th hard bases L = H+, Be2+, and Al3+, which disturb the system of bonding M Os of the cluster core. In contrast to boranes, the endohedral structure of salts with softer bases Na+, Li+, and Mg2+ corresponds to a local minimum, which is 59, 24, and 41 kcal/mol less stable than the C-3v(t) structure, r espectively, and possesses relatively high frequencies of bending vibration s. This intermediate is stabilized kinetically and can exist. The differenc es in vibrational spectra and in NMR chemical shifts of I-h and C-3v(t) iso mers are considered, along with the possibility of their identification by IR and NMR spectroscopy.