Ab Initio Study of the Hydrogen Exchange Reaction at Group 3 and 4 Metals in Comparison to That at Alkali Metals

Computations are reported at several theoretical levels for the exchange 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 transition metals have C 2 v symmetry and an energy of 8−10 kcal mol - 1 relative 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 much lower than for exchange of group 4 transition-metal hydrides (31.5−45.9 kcal mol - 1 ). The transition states of alkali-metal hydrides approximate ion pairs of the alkali-metal cations with H 3 - ion, whereas the transition-metal cases involve bonding interactions with empty d orbitals.