The reactions of Nb(C5H3RR‘)2Cl2 with Red−Al followed by hydrolysis yield Nb(C5H3RR‘)2H3 (R = R‘ = H, 1; R = H, R‘ = SiMe3, 2; R = R‘ = SiMe3, 3). These compounds react with Lewis acidic coinage cationic species, namely, [Cu(MeCN)4]PF6, AgBF4, and “Au(THT)PF6”, prepared in situ from AuCl(THT) and TlPF6 in a 2 to 1 ratio to yield the adducts [{Nb(C5H3RR‘)2H3}2M]+ (M = Cu, R = R‘ = H, 7; R = H, R‘ = SiMe3, 8; R = R‘ = SiMe3, 9; M = Ag, R = H, R‘ = SiMe3, 10; R = R‘ = SiMe3, 11; M = Au, R = R‘ = H, 12; R = H, R‘ = SiMe3, 13; R = R‘ = SiMe3, 14). Like 1, but unlike the corresponding tantalum derivatives Ta(C5H3RR‘)2H3 (R = R‘ = H, 4; R = H, R‘ = SiMe3, 5; R = R‘ = SiMe3, 6), 2 and 3 show exchange couplings in their high-field 1H NMR spectra due to a hydride tunneling phenomenon. The magnitudes of exchange couplings are larger in the cases of 2 and 3 than in the case of 1 as a result of the decrease of electron density upon increasing the number of SiMe3 substituents on the Cp ring. The addition of a Lewis acidic cation results in the observation of an AB2 pattern for the hydrides at room temperature, which splits at low temperature into an ABC one in agreement with a fluxional behavior of the cation which binds to two hydrides of each niobium center. The activation energy of these fluxional processes are close to 42−45 kJ·mol-1 in the case of Cu adducts, 37 kJ·mol-1 in the case of Ag adducts, and 40 kJ·mol-1 in the case of Au adducts. The magnitude of exchange couplings is reduced upon addition of copper cation to 1−3, is of the same order of magnitude after addition of a silver cation, and is greatly increased by addition of a gold cation. A model is proposed to explain these variations which involves two isomeric states that are close in energy, one involving two bridging and one terminal hydrides on niobium and one involving one bridging hydride and a dihydrogen molecule. A line shape analysis experiment carried out on 14 allows determination of the parameters of the classical exchange, the coupling constants at various temperatures which reach 550 Hz at 347 K, and the parameters of the quantum mechanical exchange according to our proposed model. The structure of 14 has been studied by X-ray diffraction. The structure has been solved from diffractometer data by Patterson method and refined by blocked full-matrix least squares on the basis of 3082 observed reflections to R and Rw values of 0.0346 and 0.0381, respectively. The structure shows the presence of two bridging hydrides between the niobium and gold atoms; one of them is found close to the terminal hydride.