Protonation of TpM(PR3)H-2 (M = Rh, Ir) complexes with HBF4 . Et2O or [H(Et2O)(2)][B(Ar)(4)] (Ar = 3,5-(CF3)(2)C6H3) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the H-1 NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T-l(min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond lengths of 0.88-1.11 Angstrom in the iridium complexes and 0.73-0.92 Angstrom in the rhodium complexes depending on the relative rate of the dihydrogen relational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J(H-D) or J(H-T) coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the (1)J(H-D) coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.