Molecular structure of Hf(BH4)4 investigated by quantum mechanical calculations and gas-phase electron diffraction

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Abstract
The structure of the gaseous hafnium tetrakis(tetrahydroborate) molecule, Hf(BH4)4, has been investigated by detailed quantum mechanical calculations and by analysis of its gas electron-diffraction (GED) pattern. The ground-state geometry possesses T symmetry with all of the triply-bridged BH4 groups twisted equally about the HfB–H axes. Salient structural parameters (ra distances, rα angles) deduced from the GED pattern by the SARACEN method were: r(HfB) 231.4(2), r(Hf–Hb) 221.5(7), r(B–Hb) 127.6(5), r(B–Ht) 121(1) pm, HfB–Hb 69.4(3), Hb–B–Hb 108.4(4), Hb–B–Ht 110.6(3), BHfB–Hb 166(1)°. A notable feature is the large magnitude of the HfB and Hf–Hb anharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates.