The electronic structures of the mono- acid bisammonia adducts EH3NH3 and E
H3(NH3)(2), E = B and Al, have been investigated using ab initio electronic
structure methods. Geometries were optimized at the MP2/cc-pVTZ level. Hig
her-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 energ
ies. The E-N bond dissociation energy (D,) is computed near 33 kcal/mol (E
= B) and 31 kcal/mol (E = Al), respectively. Whereas the Al-N bond energy p
ertaining 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 relat
es to electronegativity and hence the propensity of boron to engage in pred
ominantly covalent bonding, as compared with the bonding of aluminum with a
mmonia, which shows substantial electrostatic character. Neither steric fac
tors nor the participation of d-orbitals is found to play an important role
in differentiating aluminum from boron. The lesser electronegativity of th
ird-row elements appears to be the critical common feature allowing the for
mation of hypercoordinate complexes of these elements in contrast to their
second-row analogues. Consideration of some group 14 analogues acid hard/so
ft acid/base effects supports this view.