AB-INITIO STUDY OF THE COORDINATION MODES OF TETRAHYDROBORATO LIGANDS- STRUCTURE OF THE TI(BH4)3 COMPLEX

The structure of the d1-Ti(BH4)3 complex is studied by means of ab ini tio UHF calculations including correlation energy at the MP2 level. Tw enty structures which differ by the coordination mode of the BH4- liga nds are optimized through an analytical gradient method, using the dou ble-zeta basis set I. In agreement with the experimental data, the C3h (eta3, eta3, eta3) structure is found to be the most stable, a result which is confirmed by calculations with the extended basis set II. Th e geometry of the absolute minimum and the energy ordering of the othe r structures are rationalized through electron counting around the met al atom. Average bond lengths are given for each coordination mode, an d a mechanism for exchange between bridging and terminal hydrogen atom s is proposed, with an activation energy of only 5.1 kcal/mol (basis s et II).