Structures and energetics of Al(n)H(n)2- (5 < or = n < or = 12) and Al(n)H(n+2) (4 < or = n < or = 12): are alanes the borane analogues?

In this work, we report the first comparative study directly between Al(n)H(n)(2-) and B(n)H(n)(2-) (5 < or = n < or = 12), Al(n)H(n+2) and B(n)H(n+2) (4 < or = n < or = 12) covering diverse structural forms. It was shown that Al(n)H(n)(2-) each have a closo ground structure as B(n)H(n)(2-), nicely consistent with the Wade-Mingos rule. However, Al(n)H(n+2) adopt the closo-nido ground structures following the even-odd alternation in the number of Al atoms, showing distinct violation of the Wade-Mingos rule for the odd-numbered Al-atoms. Interestingly, the corresponding B(n)H(n+2) also have similar closo (even)-nido (odd) alternation. Therefore, our direct comparison showed that Al(n)H(n)(2-) (5 < or = n < or = 12) and Al(n)H(n+2) (4 < or = n < or = 12) can be viewed as the borane analogues, though not all Al(n)H(n+2) have the ability of being explained by Wade-Mingos rule. So, the analogy between alanes and boranes in form of X(n)H(n+2) should not be simply judged by the Wade-Mingos rule because of the significant influence of the additional two hydrogen atoms on the closo-structure.