Gas-phase molecular structure of (.eta.5-cyclopentadienyl)tris(tetrahydroborato)zirconium: electron-diffraction and EHMO study

Abstract

(C5H5)Zr(BHJ3 has been synthesized in excellent yield from (C5H5)ZrBr3 and LiBH, and its molecular structure in the gas phase determined by electron diffraction at a nozzle temperature of 115 OC. The results were interpreted in terms of an q5-C5H5 group and a trigonal-pyramidal arrangement of boron atoms around Zr. The infrared spectrum clearly indicates triple hydrogen bridges bctwccn thc borohydride groups and the zirconium atom; the diffraction data are fit about equally well by either double- or triple-bridge models. There is some inferential evidence in the values of the parameters, however, that suggests the triple-bridge model to bc the more likely. Values of some of the bond distances (ra/A) and bond angles (L/deg) with estimated 2a uncertainties for thc triplc-bridge model (C3 local symmetry assumed for the BH, groups) are r(Zr-B) = 2.403 (29), r(Zr-C) = 2.519 (18). r(Zr-Hb) = 2.197 (28), r(C-C) = 1.418 (2), r(B-Hb) = 1.236 (58), r(B-H,) = 1.217 (114), LQ-Zr-B (Q is the center of the cyclopcntadicnyl ring) = 115.0 (lo), LB-Zr-B = 103.4 (12), LHb-B-Hb = 103.9 (19), and LHb-B-H, = 114.6 (16). Finally, EH MO calculations are presented which support the idea that the triply bridged borohydride group can be considered to be isolobal with thc q5-C5H5 ligand.