The effect on the hydrogen exchange coupling in metallocene [(C5H5)2MH3] 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 1H 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 d4 iridium complexes that involved a dihydrogen-like transition state is also operative in the case of d0 transition metal trihydride complexes. As a matter of fact, it is the stability of the η2-H2 structure relative to the minimum energy trihydride that is the main parameter governing the magnitude of the exchange coupling.

Synthesis of stable hydride isocyanide derivatives Nb(η5-C5H4SiMe3)2(H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(η5-C5H4SiMe3)2H by thermolytic loss of H2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF3COOH leads to the η2-dihydrogen complexes [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+. NMR spectra of these H−H complexes and their monodeuterated H−D isotopomers present a single high-field resonance at room temperature. By lowering the temperature to 178 K, decoalescence of the signal was observed for the H−D complexes but not for the H−H ones. By combining DFT electronic structure calculations with a monodimensional rotational tunneling model, it has been shown that the absence of decoalescence of the H−H signal is due to the existence of a very large exchange coupling. Conversely, for the H−D isotopomer, the difference in zero point energy corresponding to two nonequivalent (H−D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange coupling, allowing decoalescence to be observed. Therefore, the H−D classical rotation and the quantum exchange processes will not be practically observed for this complex, whereas only the classical process for the H−H species is quenched out on the NMR time scale.

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