AB-INITIO STUDY OF THE COORDINATION MODES OF TETRAHYDROBORATO LIGANDS - STRUCTURE AND REACTIVITY OF THE AL(BH4)3 COMPLEX

Aluminum borohydride is studied by means of ab initio calculations taking into account correlation energy. For the (eta2,eta2,eta2) structure, the rotational barrier associated with the conrotation of the three BH4- groups around the Al-B axis is found to be very low (0.2 kcal.mol-1). A rationalization based on an orbital analysis is given. Changing the coordination mode (eta2 --> eta3) of one borohydride group leads to the (eta2,eta2,eta3) structure which is the transition state of the hydrogen exchange mechanism. This is a low energy process (2.2 kcal.mol-1) which explains that the coalescence could not be experimentally observed. Another change in the coordination mode (eta2 --> eta1) of one borohydride group leads to the (eta1,eta2,eta2) structure which is the transition state of the dissociation mechanism Al(BH4)3 --> AlH(BH4)2 + BH3. This is a high-energy process (24.8 kcal.mol-1) probably because the products are electronically deficient.