QUANTUM-MECHANICAL HYDROGEN-EXCHANGE COUPLING IN [(C2H5)IR(L)H-3](- ACOMBINED AB-INITIO TUNNELING DYNAMICS STUDY() COMPLEXES (L=PH3, CO) )

Quantum mechanical hydrogen exchange coupling in [(C5H5)Ir(L)H-3](+) c omplexes (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 trihydr ide complexes may exchange a pair of hydrogens through a tunneling pat h which involves a transition state with a eta(2)-H-2 structure. The e nergy 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, re spectively. These values are in satisfactory agreement with experiment al results. Temperature dependence of the exchange couplings is also a nalyzed and shown to be in accordance with experiments. Finally, the e ffect of the substitution of one or both of the exchanging hydrogens b y deuterium is considered.