Cationic trihydride complexes of the form [(eta-C(5)R(5))Ir(L)H-3]BF4 (R = H, Me; L = various phosphines) have been studied. The H-1 NMR spectra of these complexes at low temperature display line patterns in the hydride region consistent with AB(2)X or A(2)BX spin systems (X = P-31). The values for the H-A-H-B coupling constant (J(AB)) derived by computer simulation of the observed spectra are large, ranging from 20-830 Hz. In general, J(AB) is inversely proportional to the basicity of the ligand L and strongly temperature dependent. These unusual coupling constants have been attributed to quantum mechanical exchange coupling of the hydride ligands. All of the complexes have been partially deuterated and tritiated at the hydride sites and studied by both H-1 and H-3 NR IR spectroscopy. In contrast to J(AB), the values of J(HT) and J(TT) are independent of temperature. The observed values for J(HT) have been used to ascertain the contribution of the magnetic H-H coupling to J(AB). The contributions of the exchange coupling to J(AB) have been derived and the corresponding temperature dependency accurately modeled. Significant isotope effects on the values of J(AB) and the hydride chemical shifts were observed upon tritium and deuterium substitution. The barriers for thermally activated hydride site exchange have also been determined. No appreciable kinetic isotope effects on the thermally activated rearrangement process were observed upon substitution of D and T into the hydride sites. These results are interpreted in terms of a new two-dimensional model for quantum mechanical exchange coupling of the hydrides in these cationic complexes.