Molecular structure of gaseous titanium tris(tetrahydroborate), Ti(BH4)3: experimental determination by electron diffraction and molecular orbital analysis of some Ti(BH4)3 derivatives

Abstract

The structure of gaseous titanium tris(tetrahydroborate) has been determined by analysis of the electron diffraction patterns due to the molecules Ti(BH₄)₃ and Ti(BD₄)₃. The molecule has thus been shown to possess three tridentate BH₄ groups generating a structure with C_3h symmetry overall, implying nine-fold co-ordination of the titanium atom and a planar TiB₃ skeleton. With the assumption of local C_3v symmetry for each Ti(BD₄) fragment, salient structural parameters (r_a) are as follows: r(Ti ⋯ B) 217.5(0.4), r(Ti–D_b) 198.4(0.5), r(B–D_b) 127.6(0.5) and r(B–D_t) 116.6(1.3) pm; and within each Ti(BD₄) fragment D_b–Ti–D_b 60.1(0.7) and D_b–B–D_b 102.4(0.6)°. Such a structure is consistent with the IR and UV photoelectron spectra of the molecule in the gaseous or matrix-isolated states, but aggregation occurs in the condensed phases (e.g. in solution in a non-polar solvent or in the solid state). The structural properties of the Ti(BH₄)₃ molecule are compared with those of related tetrahydroborate derivatives, and the factors influencing these properties are assessed in terms of elementary symmetry and perturbation-theory arguments, supported by extended Hückel molecular orbital calculations.