Low-temperature protonation of compounds Cp*MH5(PMe3) (M = Mo, 1; W, 2) by HBF4·-Et2O in CD2Cl2 or CDFCl2 affords the thermally unstable \"hexahydride\" derivatives [Cp*MH6(PMe3)+ (M = Mo, 3; W, 4). The corresponding protonation of 1- and 2-d5 affords 3- and 4-d5, respectively. The △σ on going from H6 to HD5 is small for both compounds, but positive for 3 and negative for 4, and no isotopic perturbation of resonance (IPR) is observed. The T1min at 400 MHz for [Cp*MH6(PMe3)+ apparently doubles on going from Mo to W (52 ms for 3 and approximately 100 ms for 4). Optimized geometries at the restricted Hartree-Fock (RHF) and second-order Møller-Plesset (MP2) levels and energy calculations at higher levels of theory show that these complexes are dihydrogen complexes [Cp*M(H2)(H)4(PR3)]+. The theoretical determination of a dihydrogen complex is consistent with the fact that the experimental T1min values lie within the expected range for dihydrogen complexes. Examination of the potential energy surface at the MP2 level gives two mechanisms for hydride motion: exchange through a \"trihydrogen anion\" transition state and rotation of the dihydrogen ligand about its axis. The barrier for the hydride exchange (∼4 kcal/mol) is consistent with the inability to decoalesce the proton NMR signal.