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 (T1d, No. 215) with a unit cell of dimensions a = 5.827 (4) A. Full-matrix least-squares refinement gave a final value of R(F02) = 0.103 for 71 unique reflections with F02 > (σF02). The molecular structure consists of monomelic Hf(BH4)4 units having rigorously Td symmetry and tridentate tetrahydroborate coordination. Important bond distances are Hf-B = 2.281 (8) A, Hf-H(bridging) = 2.130 (9) A, B-H(bridging) = 1.235 (10) A, and B-H(terminal) = 1.150 (19) A. 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) A longer and Hf-H(bridging, average) 0.035 (9) A shorter in the latter molecule.