  Usually the ions of a particular element in solids are in positive or
negative oxidation states, depending upon the chemical environment. It is
highly unusual for an atom having both positive as well as negative oxidation
states simultaneously within the same structural framework in a particular
compound. Our structural and chemical bonding analyses show that the hydrogen
ions in the transition metal ammine borohydrides (TMABHs) with the chemical
formula M(BH4)2(NH3)2 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have both
positive and negative oxidation states. Our detailed analyses confirm the
amphoteric behaviour of hydrogen where hydrogen closer to boron is in negative
oxidation state and that closer to nitrogen is in the positive oxidation state.
From the spin polarised van der Waals interaction included calculation, we
found that TMABHs has antiferromagnetic ordering as ground state. Our nudged
elastic band calculation show that the migration barrier for ammonia diffusion
in these materials are more than 1 eV and hence these compounds may be used for
energy storage applications since they have high weight percentage of hydrogen.
The presence of amphoteric behaviour of hydrogen in TMABHs has implication in
designing volume efficient hydrides for hydrogen storage applications.
