Addition of ammonia to AlH3 and BH3. Why does only aluminum form 2:1 adducts?

The electronic structures of the mono- and bisammonia adducts EH3NH3 and EH3(NH3)2, E = B and Al, have been investigated using ab initio electronic structure methods. Geometries were optimized at the MP2/cc-pVTZ level. Higher-level correlated methods (MP4(SDTQ), QCISD(T), CCSD(T)), as well as the G2 and CBS-Q methods, were used to obtain accurate bond dissociation energies. The E-N bond dissociation energy (De) is computed near 33 kcal/mol (E = B) and 31 kca/mol (E = Al), respectively. Whereas the Al-N bond energy pertaining to the second ammonia molecule in AlH3(NH3)2 is 11-12 kcal/mol, only a transition-state structure may be located for the species BH3(NH3)2. We analyze factors which may distinguish Al from B with respect to the formation of stable bisamine adducts. The most significant difference relates to electronegativity and hence the propensity of boron to engage in predominantly covalent bonding, as compared with the bonding of aluminum with ammonia, which shows substantial electrostatic character. Neither steric factors nor the participation of d-orbitals is found to play an important role in differentiating aluminum from boron. The lesser electronegativity of third-row elements appears to be the critical common feature allowing the formation of hypercoordinate complexes of these elements in contrast to their second-row analogues. Consideration of some group 14 analogues and hard/soft acid/base effects supports this view.