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

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 Hf⋯B–H axes. Salient structural parameters (r_a distances, r_α angles) deduced from the GED pattern by the SARACEN method were: r(Hf⋯B) 231.4(2), r(Hf–H_b) 221.5(7), r(B–H_b) 127.6(5), r(B–H_t) 121(1) pm, Hf⋯B–H_b 69.4(3), H_b–B–H_b 108.4(4), H_b–B–H_t 110.6(3), B⋯Hf⋯B–H_b 166(1)°. A notable feature is the large magnitude of the Hf⋯B and Hf–H_b anharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates.