Stereochemical nonrigidity in solid zirconium and hafnium tetrakistetrahydroborates. Evidence for two dynamic intramolecular rearrangement processes in covalent tridentate tetrahydroborates

Proton nuclear magnetic resonance studies of stereochemical dynamics in Zr(BH ₄) ₄ and Hf(BH ₄) ₄ have been carried out in the solid state. Variable temperature investigations were conducted in both continuous wave and pulsed modes. Two intramolecular motional processes were identified for each complex and the nature of these processes was interpreted on the basis of effective second moment calculations in both diffusive and positional jump regimes. The higher temperature process is assigned to rapid intramolecular bridge-terminal hydrogen exchange, E _a(Zr) = 5.2 ± 0.3 (ΔG ^≠ = 8.1 at 214 K) 1; E _a(Hf) = 8.4 ± 0.3 (ΔG ^≠ = 7.3 at 200 K) kcal/mol. The lower temperature motion is suggested to involve rapid threefold rotation about the ligand M-B-H (terminal) axis with E _a(Zr) = 5.4 ±0.2 (ΔG ^≠ = 4.3 at 124 K) 1; E _a(Hf) = 4.6 ±0.2 (ΔG ^≠ = 4.4 at 133 K) kcal/mol.