GAS-PHASE MOLECULAR-STRUCTURE OF (ETA-5-C5H5)ZR(BH4)3 - ELECTRON-DIFFRACTION AND EHMO STUDY

(C5H5)Zr(BH4)3 has been synthesized in excellent yield from (C5H5)ZrBr3 and LiBH4 and its molecular structure in the gas phase determined by electron diffraction at a nozzle temperature of 115-degrees-C. The results were interpreted in terms of an eta-5-C5H5 group and a trigonal-pyramidal arrangement of boron atoms around Zr. The infrared spectrum clearly indicates triple hydrogen bridges between the 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 be the more likely. Values of some of the bond distances (r(a)/angstrom) and bond angles (angle a/deg) with estimated 2-sigma uncertainties for the triple-bridge model (C3v local symmetry assumed for the BH4 groups) are r(Zr-B) = 2.403 (29), r(Zr-C) = 2.519 (18), r(Zr-H(b)) = 2.197 (28), r(C-C) = 1.418 (2), r(B-H(b)) = 1.236 (58), r(B-H(t)) = 1.217 (114), angle Q-Zr-B (Q is the center of the cyclopentadienyl ring) = 115.0 (10), angle B-Zr-B = 103.4 (12), angle H(b)-B-H(b) = 103.9 (19, and angle H(b)-B-H(t) = 114.6 (16). Finally, EHMO calculations are presented which support the idea that the triply bridged borohydride group can be considered to be isolobal with the eta-5-C5H5 ligand.