Theoretical study of the hydrogen exchange coupling in the metallocene trihydride complexes [(C5H5)(2)MH(3)](n+) (M=Mo, W, n=1; M=Nb, Ta, n=0)

The effect on the hydrogen exchange coupling in metallocene [(C5H5)(2)MH(3)](n+) complexes under the substitution of the transition metal M is theoretically analyzed using a simple methodology that requires a modest number of ab initio electronic energy calculations that are used in a one-dimensional tunneling model within a basis set method. Concretely, the cases M = Mo, W (n = 1) and M = Nb, Ta (n = 0) whose hydrogen exchange couplings have been experimentally measured through the corresponding H-1 NMR spectra are considered. Our results of the exchange couplings at different temperatures for the considered cases are in satisfactory agreement (within the correct order of magnitude) with experimental results. This agreement seems to confirm that the mechanism we previously established for some formally d(4) iridium complexes that involved a dihydrogen-like transition state is also operative in the case of d(0) transition metal trihydride complexes. As a matter of fact, it is the stability of the eta(2)-H-2 structure relative to the minimum energy trihydride that is the main parameter governing the magnitude of the exchange coupling.