Unusual and Conventional Dative Bond Formation by s² Lone Pair Donation from Alkaline Earth Metal Atoms to BH₃, AlH₃, and GaH₃

Abstract

Using ab initio electronic structure methods with flexible atomic orbital basis sets, we examined the nature of the bonding arising from donation of an ns2 electron pair on an alkaline earth atom (Mg or Ca) into a vacant n′p orbital on the group 13 atom of BH3, AlH3, or GaH3. We also examined what happens when an excess electron is attached to form corresponding molecular anions. Although the geometries of MgBH3, MgAlH3, MgGaH3, and CaBH3 are found to be much as one would expect for datively bound molecules, CaAlH3 and CaGaH3 were found to have very unusual geometries in that their Al–H or Ga–H bonds are directed toward the Ca atom rather than away, as in the other compounds. Internal electrostatic Coulomb attractions between the partially positively charged Ca center and the partially negatively charged H centers were suggested as a source of these unusual geometries. The other novel finding is that the electron affinities (EAs) of all six M′–MH3 species lie in the 0.7–1.0 eV range, which is suggestive of ionic electronic structures for the neutrals even though the partial charges on the alkaline earth centers are as low as 0.3 atomic units. Partial positive charge on the alkaline earth atoms combined with substantial electron affinities of the BH3, AlH3, and GaH3 groups, but only when distorted from planar geometries, were suggested to be the primary contributors to the large EAs.

Role of Dynamical Electron Correlation in the Differences in Bonding between CaAlH3 and MgAlH3

“…The fully optimized C 3 v geometries obtained at the CCSD­(T) level using aug-cc-pVTZ and aug-cc-pVQZ basis sets (see Table ) reproduce a key finding of Anusiewicz et al: in contrast to the optimal geometry for MgAlH 3 which has a fairly conventional small positive value for θ of ca . +4°, the corresponding angle for CaAlH 3 is significantly negative ( ca .…”

“…All vibrational frequencies were found to be real, confirming that we had indeed located local minima. The resulting geometric parameters turned out, unsurprisingly, to be similar to those reported by Anusiewicz et al To enable fair comparisons of the bonding situations in MgAlH 3 and CaAlH 3 , we also optimized model geometries for both systems, with θ fixed at −35° for MgAlH 3 and at +4° for CaAlH 3 . The various CCSD­(T) geometry optimizations, which were all carried out using MOLPRO, − were subsequently repeated using instead standard aug-cc-pVQZ basis sets, as stored internally by MOLPRO.…”

“…The basic geometric arrangement that would then be anticipated for such a C 3 v complex is as shown in Figure , in which θ, the deviation of the M′MH bond angle from 90°, is expected to be relatively small and positive, perhaps on the order of +4°, so that the H atoms are directed away from M′. This is indeed what has been found when M′ is Mg but Anusiewicz et al observed for the interaction of Ca with AlH 3 or GaH 3 that the resulting neutral C 3 v complexes have values of θ that are significantly negative, typically on the order of −35°, so that the H atoms are instead directed toward M′; see Figure and Table . (Instead of this easily visualized “umbrella angle” θ, we could of course have specified these geometries in terms of the HMHH dihedral angle, in an analogous fashion to interesting previous work on stable anions of M′BH 3 systems …”

Collective interactions among organometallics are exotic bonds hidden on lab shelves

“…The Δ∠M‒A‒X angle is defined as the difference between ∠M‒A‒X angle and a right angle as defined in ref. 9 . a Data obtained from BS-DFT calculations.…”

“…One may speculate that aluminum analogs of NaBH 3 − ([M n+ AlH 3 ] 2− n ), on one hand, and their carbon analogs ([M n+ CR 3 ] 1− n ) on the other hand, might form similar bonds. Although the former aluminum clusters are at present a mere computational curiosity 9 , the latter carbon-based group includes a large number of well-known organometallic reagents, including Grignard and organolithium reagents. Are the closest relatives of the Na‒B bond in NaBH 3 − already on the shelves of the chemistry labs but unnoticed by theoretical chemists?…”