Synthesis, X-ray crystal structure, NMR characterization and theoretical calculations on [Cp2Ta(eta(2)-H-2)(CO)](+), the first thermally stable group5 dihydrogen complex

Protonation of Cp2TaH(CO) (Cp = C5H5, 1a; C5H4But, 1b) by HBF4 . Et2O at -7 8 degreesC in CH2Cl2 affords [Cp2TaH2(CO)]BF4 (2, 3) as mixtures of 2 isome rs. The minor ones (2a, 2b) contain the known trans-dihydride [Cp2TaH2(CO)] (+) cations whereas the major ones (3a, 3b) are [Cp2Ta(eta (2)-H-2)(CO)]BF4 , the first group 5 dihydrogen complexes. The crystal structure of the anal ogous complex 3a . BArf(4) recorded at 120 K confirms the presence of the c oordinated dihydrogen ligand, which displays an H-H separation of 1.09(2) A ngstrom in agreement with distances calculated from NMR data. Protonation o f Cp2TaH2(SiMe2Ph) by (Et2O)(2) . HBArf(4) does not lead to an analogous si lane derivative but to the new dinuclear complex [(Cp2TaH2)(2)(mu -H)](BArf (4)). Variable temperature NMR studies were carried out on the dihydrogen c omplex [Cp2Ta(H-2)(CO)](+) (3) and its isotopomers. The high field signal o f [Cp2Ta(HD)(CO)](+) (3-d) shows a decoalescence at 208 K in both H-1 and D -2 NMR, which allows us to calculate the barrier to rotation of HD (9.6 kca l mol(-1)). The absence of decoalescence in the signal of 3 down to 173 K a nd the absence of a large kinetic isotope effect for the classical rotation of H-2 were demonstrated. These results are understood in terms of the pre sence 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 l igand is only coordinated through sigma -donation from the H-H bond. The an alogous 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.