PROPERTIES OF SMALL GROUP-IIIA HYDRIDES INCLUDING THE CYCLIC AND PENTACOORDINATE STRUCTURES OF TRIALANE (AL3H9) AND TRIGALLANE (GA3H9) - CAN DIALANE BE ISOLATED

The structure of small group IIIA hydrides up to the trimers are studied using the self-consistent field (SCF), single and double excitation configuration interaction (CISD), and single and double excitation coupled cluster (CCSD) methods in conjunction with a double-zeta plus polarization basis set (DZP). The dimerization energy for the unknown dialane is found to be significantly greater than that for the recently synthesized digallane. Unlike the analogous boron compound, cyclic trialane(9) and trigallane(9) incorporate planar six-membered rings according to the DZP SCF predictions. Acyclic trimer structures with pentacoordinated heavy atoms are also considered. The pentacoordinate trialane(9) is nearly isoenergetic with the cyclic structure, and the pentacoordinate triborane(9) is only slightly higher in energy than the cyclic triborane. This new pentacoordinated triborane may be involved in the pyrolysis of diborane and may be the key to the resolution of a dispute concerning the kinetics of this process.