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(BH/sub 4/)/sub 4/ by single-crystal neutron diffraction at 110 K. The compound crystallizes in the cubic space group P43m (T/sub d//sup 1/, No. 215) with a unit cell of dimensions a = 5.827 (4) angstrom. Full-matrix least-squares refinement gave a final value of R(F/sub 0//sup 2/) = 0.103 for 71 unique reflections with F/sub 0//sup 2/ > sigma(F/sub 0//sup 2/). The molecular structure consists of monomeric Hf(BH/sub 4/)/sub 4/ units having rigorously T/sub d/ symmetry and tridentate tetrahydroborate coordination. Important bond distances are Hf-B = 2.281 (8) angstrom, Hf-H(bridging) = 2.130 (9) angstrom, B-H(bridging) = 1.235 (10) angstrom, and B-H(terminal) = 1.150 (19) angstrom. Important bond angles are Hf-H(bridging)-B = 80.6 (6)/sup 0/, H(bridging)-B-H(bridging) = 105.8 (6)/sup 0/, and H(bridging)-B-H(terminal) = 112.9 (10)/sup 0/. It is seen that the internal BH/sub 4//sup -/ metrical parameters do not differ significantly from those in Hf(CH/sub 3/C/sub 5/H/sub 4/)/sub 2/(BH/sub 4/)/sub 2/, in which the tetrahydroborate ligands are coordinated in an unsymmetrical bidentate fashion. However, Hf-B is 0.17 (1) angstrom longer and Hf-H(bridging, average) 0.035 (9) angstrom shorter in the latter molecule.