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...