A Spectroscopic and Neutron Diffraction Investigation of Structure, Bonding, and Molecular Dynamics in Bis(methylcyclopentadienyl)hafnium Bis(tetrahydroborate), (η⁵-Ch₃C₅H₄)₂ Hf(BH₄)₂

The complex (CH3C5H4)2 Hf(BH4)2 can be prepared by the reaction of (CH3C5H4)2 HfCl2 with LiBH4 in benzene. The bonding, molecular dynamics, and structure of this compound have been studied by infrared and Raman vibrational spectroscopy, by dynamic nuclear magnetic resonance spectroscopy, and by single-crystal neutron diffraction. Vibrational spectra reveal the perturbation of the boron-containing ligand from a free BH4- ion to be less in this complex than in most covalent tetrahydroborate transition metal complexes. Exchange of bridge and terminal BH4- hydrogen atoms is rapid on the 90-MHz 1H NMR time scale down to -155 °C; thus, ΔG‡ for the interchange process is less than ca. 4.9 kcal/mol. The compound crystallizes from toluene/pentane in the monoclinic space group C2/c with four molecules in a unit cell of dimensions a = 14.566 (3), b = 6.839 (1), c = 13.959 (3) Å, and β = 102.31 (2)°. Full-matrix least-squares refinement gave a final value of R(Fo2) = 0.107 for 1022 reflections. The molecular structure consists of monomeric (η5-CH3C5H4)2 Hf(BH4)2 units having C2 symmetry and bidentate tetrahydroborate ligands. The ring centroid-Hf-ring centroid angle is 128.5 (3)°. The average Hf-C (ring) bond distance is 2.491 (9) Å. The coordination of each tetrahydroborate group is markedly unsymmetrical with crystallographically independent Hf-H (bridge) distances of 2.069 (7) and 2.120 (8) Å. The lengths of the two B-H (bridge) bonds mirror, inversely, the Hf-H distances: 1.255 (9) and 1.208 (13) Å. The B-H (terminal) distances are 1.186 (16) and 1.198 (15) Å, and the Hf-B distance is 2.553 (6) Å. Important bond angles are B-Hf-B = 101.4 (3)°, H (bridge)-B-H (bridge) = 108.4 (6)°, and H (terminal)-B-H (terminal) = 115.4 (10)°. The anisotropy of the hydrogen atom thermal parameters within the tetrahydroborate groups may reflect motion along the bridge-terminal hydrogen interchange reaction coordinate. The nature of the (CH3C5H4)2 Hf-tetrahydroborate interaction as observed in the vibrational spectra and molecular structure can be understood in terms of those bonding factors which govern ligand-to-metal charge donation.