CRYSTAL AND MOLECULAR-STRUCTURE OF AL2(O-T-BU)6 - COMMENTS ON THE EXTENT OF M-O-PI BONDING IN GROUP-6 AND GROUP-13 ALKOXIDES

The reaction between Al2(NMe2)6 and excess t-BuOH yields Al2(O-t-Bu)6, which crystallizes in the space group PlBAR with a = 9.946 angstrom, b = 9.755 angstrom, c = 16.332 angstrom, alpha = 88.89-degrees, beta = 73.81-degrees, gamma = 88.89-degrees, and Z = 2. The molecule is pseudotetrahedral about each Al center and exhibits terminal Al-O bond lengths that are ca. 0.16 angstrom shorter than the sum of their atomic radii. The difference between the terminal Al-X bond lengths in Al2Me6 and Al2(O-t-Bu)6 amounts to 0.28 angstrom, the same bond length differential as found for the Mo-X distances in Mo2R6 and Mo2(OR)6 complexes. Both ab initio and Fenske-Hall-type MO calculations were performed on the model complexes M2(OH)6 (M = B, Al, Mo) in order to compare the M-O bonding in group 6 and group 13 alkoxides. For M = B and Al, the extent of M-O pi bonding was found to be low and concentrated primarily on the terminal M-O interactions. The inclusion of Al 3d orbitals did not significantly increase the Al-O pi bond orders. The ionic contribution calculated for the Al-O interactions was found to be significant and the major cause of the short Al-O distances. For M = Mo, the short Mo-O bond lengths were found to be the result of significant O(p-pi)-Mo(d-pi) bonding, with less of an ionic contribution. Ab initio calculations were also performed on the model three-coordinate monomeric systems H2Al(OH), H2AlMe, H2B(OH), and H2BMe at the 6-31G* level. The optimized geometries showed the Al-O bond to be 0.28 angstrom shorter than the Al-C bond in these systems. The pi bond orders were found to be only 0.26 for M = Al and 0.37 for M = B, indicating a significant degree of ionic character responsible for the short M-O bond lengths. Ab initio calculations on the model system H4MoO showed a high degree of triple bond character in the Mo-O interaction.