Several dihydrogen complexes of ruthenium of the form [Cp/Cp*Ru(P-P)H-2](+) (P-P = chelating diphosphine ligand) have been prepared by reaction of the corresponding neutral chloride complexes with H-2 in the presence of NaB(ArF)(4). Treatment with D-2 or T-2 gas leads to incorporation of deuterium or tritium in the dihydrogen ligand. Measurement of the resulting H-D and H-T couplings as a function of the temperature and magnetic field gives results consistent with computational studies which predict that the H-H bond distance will increase with temperature and will be significantly shortened by isotopic substitution. The degree of the observed temperature dependence is found to be a critical function of the ancillary ligand set.