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

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.