Metrical characterization of tridentate tetrahydroborate ligation to a transition-metal ion. Structure and bonding in Hf(BH₄)₄ by single-crystal neutron diffraction

This paper reports the molecular structure of the transition-metal tetrahydroborate complex Hf(BH₄)₄ by single-crystal neutron diffraction at 110 K. The compound crystallizes in the cubic space group P43m (T_d¹, No. 215) with a unit cell of dimensions a = 5.827 (4) Å. Full-matrix least-squares refinement gave a final value of R(F_o²) = 0.103 for 71 unique reflections with F_o² > σ(F_o²). The molecular structure consists of monomeric Hf(BH₄)₄ units having rigorously T_d symmetry and tridentate tetrahydroborate coordination. Important bond distances are Hf-B = 2.281 (8) Å, Hf-H(bridging) = 2.130 (9) Å, B-H(bridging) = 1.235 (10) Å, and B-H(terminal) = 1.150 (19) Å. Important bond angles are Hf-H(bridging)-B = 80.6 (6)°, H(bridging)-B-H(bridging) = 105.8 (6)°, and H(bridging)-B-H(terminal) = 112.9 (10)°. It is seen that the internal BH₄^- metrical parameters do not differ significantly from those in Hf(CH₃C₅H₄)₂(BH₄) ₂) in which the tetrahydroborate ligands are coordinated in an unsymmetrical bidentate fashion. However, Hf-B is 0.27 (1) Å longer and Hf-H(bridging, average) 0.035 (9) Å shorter in the latter molecule.