Quantum mechanical hydrogen exchange coupling in [(C5H5)Ir(L)H3]+ complexes (L = PH3, CO) has been studied by combining the construction of ab initio potential energy surfaces with a tunneling model using a basis set method. Our results show that these transition-metal trihydride complexes may exchange a pair of hydrogens through a tunneling path which involves a transition state with a η2-H2 structure. The energy barriers for L = PH3 and CO are respectively 14.1 and 10.7 kcal/mol, leading to exchange coupling constants of 112.3 and 1279.5 Hz, respectively. These values are in satisfactory agreement with experimental results. Temperature dependence of the exchange couplings is also analyzed and shown to be in accordance with experiments. Finally, the effect of the substitution of one or both of the exchanging hydrogens by deuterium is considered. © 1995, American Chemical Society. All rights reserved.