Synthesis, X-ray crystal structure, NMR characterization and theoretical calculations on [Cp2Ta(η2-H2)(CO)]+, the first thermally stable group 5 dihydrogen complex

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Protonation of Cp2TaH(CO) (Cp = C5H5, 1a; C5H4But, 1b) by HBF4·Et2O at −78 °C in CH2Cl2 affords [Cp2TaH2(CO)]BF4 (2, 3) as mixtures of 2 isomers. The minor ones (2a, 2b) contain the known trans-dihydride [Cp2TaH2(CO)]+ cations whereas the major ones (3a, 3b) are [Cp2Ta(η2-H2)(CO)]BF4, the first group 5 dihydrogen complexes. The crystal structure of the analogous complex 3a·BArf4 recorded at 120 K confirms the presence of the coordinated dihydrogen ligand, which displays an H–H separation of 1.09(2) A in agreement with distances calculated from NMR data. Protonation of Cp2TaH2(SiMe2Ph) by (Et2O)2 ·HBArf4 does not lead to an analogous silane derivative but to the new dinuclear complex [(Cp2TaH2)2(μ-H)](BArf4). Variable temperature NMR studies were carried out on the dihydrogen complex [Cp2Ta(H2)(CO)]+ (3) and its isotopomers. The high field signal of [Cp2Ta(HD)(CO)]+ (3-d) shows a decoalescence at 208 K in both 1H and 2D NMR, which allows us to calculate the barrier to rotation of HD (9.6 kcal mol−1). The absence of decoalescence in the signal of 3 down to 173 K and the absence of a large kinetic isotope effect for the classical rotation of H2 were demonstrated. These results are understood in terms of the presence of very large exchange couplings in a non-rotating dihydrogen ligand. The large barrier of rotation for the dihydrogen ligand in 3 was shown by DFT calculations to arise from a transition state in which the dihydrogen ligand is only coordinated through σ-donation from the H–H bond. The analogous phosphite and phosphine complexes {Cp2TaH2[P(OMe)3]}+ (4) and [Cp2TaH2(PMe2Ph)]+ (5) were shown to be cis dihydrides, in agreement with DFT calculations on a model compound, to display exchange couplings in NMR and no isotope effect for the classical exchange of the hydride ligands.