Bis(η⁵-cyclopentadienyl)vanadium Tetrahydroborate. A Covalent Organotransition Metal Borohydride with Unusual Spectroscopic and Dynamic Properties

The synthesis and spectroscopic (vibrational, nmr) characterization of (η⁵-C₅H₅)VBH₄ are described. In comparison to (η⁵-C₅H₅)TiBH₄, both (C₅H₅)₂VBH₂ and (C₅H₅)NbB₄, which also have bidentate MH₂BH₂ ligation, appear to be more covalent and show considerable weakening of B-H bridging bonds and possibly some strengthening of M-H bonds in the vibrational spectra. In addition, the free energy barrier to bridge-terminal hydrogen interchange is surprisingly high for the vanadium compound (ΔG^‡ ≈ 7.6 ± 0.3 keal/mol) and the fluxional behavior can be slowed on the nmr time scale. The permutation process is too rapid to slow in the niobium compound, and the reasons for the difference in rate are discussed in terms of the proposed rearrangement mechanism and the size of the metal ion.