The previously reported cationic dihydride complexes (PP3)MH(2)(+) (M = Co, Ph and Ir; PP3 = P(CH(2)CH(2)PPh(2))(3)) have been prepared using improved synthetic methods. Variable-temperature H-1 and P-31 NMR spectra of these complexes reveal complex dynamic behavior. The hydride region H-1 NMR spectra have been accurately simulated at all temperatures using a simple site permutation model after taking into consideration the opposite signs of the cis and trans H-P coupling constants. Partial deuteration of the hydride ligands in the rhodium and cobalt complexes is achieved by exposure to D-2. In the partially deuterated samples, no evidence is found for a bound dihydrogen ligand, but the involvement of a dihydrogen species in the dynamic process which interchanges the two hydride positions remains a mechanistic possibility, as indicated by a kinetic isotope effect k(H)/k(D) = 1.3(1) The partially deuterated samples exhibit large and temperature-dependent isotope effects on the H-1 NMR chemical shifts observed for the hydride resonances, which are attributed to isotopic perturbation of resonance, This arises from non-statistical occupation of the two different hydride sites and also leads to perturbation of the averaged H-P coupling constants. Similar observations have been made for the neutral iron complex (PP3)FeH2.