Periodic Trends Manifested through Gas-Phase Generation of Anions Such as [AlH4]-, [GaH4]-, [InH4]-, [SrH3]-, [BaH3]-, [Ba(0)(η2-O2CH)1]-, [Pb(0)H]-, [Bi(I)H2]-, and Bi- from Formates.

Abstract

Metal-hydride anions of main group elements, such as BaH3– and InH4–, were generated by dissociating formate adducts of the respective metal formates. Upon activation, these adducts fragment by formate-ion ejection or by decarboxylation. For adducts of alkali-metal formates, the formate-ion ejection is the preferred pathway, whereas for those of alkaline-earth and group 13–15 metals, the expulsion of CO2 is the more favorable pathway. Decarboxylation is deemed to yield a metal–hydrogen bond presumably by a hydride transfer to the metal atom. For example, the decarboxylation of Al(η-OCOH)4– and Ga(η-OCOH)4– generated AlH4– and GaH4–, respectively. The initial fragment-ion with a H–M bond formed in this way from adducts of the heavier metals of group 13 (Ga, In, and Tl) undergo a unimolecular reductive elimination, ascribable to the “inert-pair” effect, to lower the metal-ion oxidation state from +3 to +1. As group 13 is descended, the tendency for this reductive elimination process increases. PbH3–, generated from the formate adduct of lead formate, reductively eliminated H2 to form PbH–, in which Pb is in oxidation state zero. In the energy-minimized structure [H–Pb(η2-H2)]−, proposed as an intermediate for the process, a H2 molecule is coordinated with PbH– as a dihapto ligand. The formate adducts of strontium and barium produce monoleptic ions such as [M(0)(η2-O2CH)1]−, in which the formate ion is chelated to a neutral metal atom. The bismuth formate adduct undergoes a double reductive elimination process whereby the oxidation state of Bi is reduced from +3 to +1 and then to −1. Upon activation, the initially formed [H–Bi–H]− ion transforms to an anionic η2-H2 complex, which eliminates dihydrogen to form the bismuthide anion (Bi–).