α,β‐Unsaturated and Saturated Derivatives of Be, Mg, and Ca: Are They Carbon or Metal Acids in the Gas Phase?

Abstract

The gas-phase acidity of R--XH (R=H, CH(3), CH(2)CH(3), CH==CH(2), C[triple chemical bond]CH; X=Be, Mg, Ca) alkaline-earth-metal derivatives has been investigated through the use of high-level CCSD(T) calculations by using a 6-311+G(3df,2p) basis set. BeH(2) is a stronger acid than BH(3) and CH(4) for two concomitant reasons: 1) the dissociation energy of the Be--H bond is smaller than the dissociation energies of the B--H and C--H bonds, and 2) the electron affinity of BeH(.) is larger in absolute value than those of BH(2) (.) and CH(3) (.). The acidity also increases on going from BeH(2) to MgH(2) due to these two same factors. Quite importantly, despite the fact that the X--H bonds in the R--XH (X=Mg, Ca) derivatives exhibit the expected X(delta+)--H(delta-) polarity, they behave as metal acids in the gas phase and only Be derivatives behave as carbon acids in the gas phase. The ethylberyllium hydride exhibits an unexpected high acidity compared with the methyl derivative because deprotonation of the system is accompanied by a cyclization that stabilizes the anion. Similarly to that found for derivatives that contain heteroatoms from groups 14, 15, and 16, the unsaturated compounds are stronger acids than the saturated counterparts, with the only exception of the Ca-vinyl derivative. Most importantly, among ethyl, vinyl, and ethynyl derivatives containing a heteroatom of the main group of the Periodic Table, those containing Be, Mg, and Ca are among the strongest gas-phase acids.

Are cyclopentadienylberyllium, magnesium and calcium hydrides carbon or metal acids in the gas phase?

“…It is worth noting that this prediction does not change when G3B3 theory is used instead of DFT approaches. This finding is in agreement with the behavior predicted for the corresponding methyl derivatives, 38 with the difference, that for CpBeH, the energy gap between the C-and the Be-deprotonated anions was found to be almost three times larger than that calculated for methylberyllium hydride, at the same level of theory. 38 The reason why, for the beryllium derivative, the most acidic site is a CH group of the C 5 H 5 moiety, while for the Mg and Ca analogues it is the XH (X = Mg, Ca) group, can be rationalized by means of the thermodynamic cycle shown in Scheme 1, in which the deprotonation reaction is viewed as a two step process, namely, the homolytic cleavage of the X-H or the C-H bond and the electron transfer from the H atom to the radical formed in the Scheme 1…”

“…Are the Mg and Ca containing compounds metal acids in the gas phase as it has been suggested to be the case for the alkyl, vinyl and ethynyl-derivatives? 38 The aim of this paper is to provide an answer to the aforementioned questions by means of ab initio molecular orbital and density functional theory (DFT) calculations.…”

1‐Alkenylcalcium Iodide: Synthesis and Stability

“…However, beside a few structurally characterized alkenylmagnesium derivatives,10 a single well‐defined alkenylbarium compound, namely, (tetrahydrofuran‐O)barium 2,5‐diphenyl‐3‐(1,4‐diphenylbutene‐3‐yne‐2‐ide‐1‐yl)‐4‐trimethylsilyl‐1‐phosphacyclopentadienide,11 is known and underlines that the alkenyl derivatives of the heavier alkaline‐earth metals are also stable enough for isolation under certain conditions. Therefore, we intended to expand the knowledge on such calcium‐based organometallics beyond quantum‐chemical calculations,12 anticipating complexes with an extremely high reactivity.…”

Investigating the electronic properties and structural features of MgH and of MgH− anions

“…There is therefore a level of uncertainty as to which value should be selected from experiments, although no earlier data exist for MgD − . Later calculations of the EA value for MgH − were carried out by [21] and reported also a different value for it: 0.83 eV.…”