Electronic Structure and Initial Dehydrogenation Mechanism of M(BH4)2•2NH3 (M = Mg, Ca, and Zn): A First-Principles Investigation

The electronic structure and initial dehydrogenation mechanism of M(BH4)(2)center dot 2NH(3) (M = Mg, Ca, and Zn) have been systematically studied using first-principles calculations. A detailed study of the electronic structure reveals that the metal cations in M(BH4)(2)center dot 2NH(3) play a crucial role in both suppressing ammonia emission and destabilizing the N-H/B-H bonds. The calculation results of hydrogen removal energies are in agreement with the tendency of dehydrogenation temperatures of these ammoniates, i.e., Zn(BH4)(2)center dot 2NH(3) < Mg(BH4)(2)center dot 2NH(3) < Ca(BH4)(2)center dot 2NH(3). The initial dehydrogenation of M(BH4)(2)center dot 2NH(3) is achieved by the dissociation of (N)H delta+ from NH3 and (B)H delta- atoms from BH4 groups, resulting in the formation of N-B dative bonds and the reduction of the neighboring (N)H delta+center dot center dot center dot(B)H delta- dihydrogen bonds, which accelerate the subsequent dehydrogenation.