AB-INITIO STUDY OF THE HYDROGEN-EXCHANGE REACTION AT GROUP-3 AND GROUP-4 METALS IN COMPARISON TO THAT AT ALKALI-METALS

Computations are reported at several theoretical levels for the exchan ge reaction of group 3 and 4 metal hydrides with hydrogen. At the CISD +Q level weak complexes are formed with an association energy of 0.1-3 .0 kcal mol(-1). The transition states for the exchange with group 3 t ransition metals have Ct, symmetry and an energy of 8-10 kcal mol-l re lative to the reactants. This barrier is lower than that for exchange of alkali-metal hydrides with hydrogen (15.7-21.7 kcal mol(-1)) and mu ch lower than for exchange of group 4 transition-metal hydrides (31.5- 45.9 kcal mol-l). The transition states of alkali-metal hydrides appro ximate ion pairs of the alkali-metal cations with H-3(-) ion, whereas the transition-metal cases involve bonding interactions with empty d o rbitals.