I. Demachy et F. Volatron, AB-INITIO STUDY OF THE COORDINATION MODES OF TETRAHYDROBORATO LIGANDS- STRUCTURE AND REACTIVITY OF THE AL(BH4)3 COMPLEX, Inorganic chemistry, 33(18), 1994, pp. 3965-3969
Aluminum borohydride is studied by means of ab initio calculations tak
ing into account correlation energy. For the (eta2,eta2,eta2) structur
e, 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.mo
l-1). A rationalization based on an orbital analysis is given. Changin
g the coordination mode (eta2 --> eta3) of one borohydride group leads
to the (eta2,eta2,eta3) structure which is the transition state of th
e 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(B
H4)2 + BH3. This is a high-energy process (24.8 kcal.mol-1) probably b
ecause the products are electronically deficient.