Addition of Ammonia to AlH₃and BH₃. Why Does Only Aluminum Form 2:1 Adducts?

Abstract

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.

Nucleophilicity—Periodic Trends and Connection to Basicity

“…Even neutral aluminum has this ability, in contrast to boron, [38] which is most probably the result of its enhanced tendency for electrostatic binding at the expense of covalency. [39] We also notice the parallel at the other extreme, namely binding in proton transfer. While the minimum-energy structure for [(H 2 O) 2 …”

“…fourth rows. [39] The well-known phenomenon of decreasing S N 2 reactivity upon increased alkyl substitution at the central carbon is probably more related to the same central atom size/electron-accepting properties, and probably less to the so-called steric effect (direct repulsion from the alkyl substituents) than is generally believed. [64] Relationship between nucleophilicity and basicity: In the final few paragraphs, the ongoing discussion of the relationship between basicity and nucleophilicity is taken up once more.…”

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

“…It was noted in earlier studies that the Lewis acidity of the boron trihalides increases in the order BF 3 <BCl 3 <BBr 3 ,33 which does not agree with the electronegativity increase trend for halogen substituents. For example, for the BF 3 ‐NH 3 and BCl 3 ‐NH 3 complexes, the stronger bonding occurs for the latter species 64. Brinck and co‐workers explained earlier that the greater Lewis acidity of boron in the trichloride than the trifluoride comes from the larger charge capacity for BCl 3 65.…”

Hydride‐Triel Bonds

“…Much attention has been paid to small dimer systems featuring the N⋯Y triel bond, where Y is the 13 group element (i.e., a triel element) as B, Al or Ga . In particular, dimers where or BR 3 (R = F, Cl) acts as Lewis acid have been investigated.…”

“…At the same time, they pointed out the importance of charge transfer from the Lewis base to the BR 3 Lewis acid. It seems that dimers featuring a triel bond of the N⋯Y type with Y being a triel element other than boron are much less often investigated, in particular by theoretical methods . Apart from small molecules with nitrogen participating in the X⋯Y triel bond, also those featuring either the carbon or the oxygen atom or, in particular, σ or π bonds have been investigated.…”