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

Metal-hydride anions of main group elements, such as BaH 3 - and InH 4 - , 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 CO 2 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 AlH 4 - and GaH 4 - , 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. PbH 3 - , generated from the formate adduct of lead formate, reductively eliminated H 2 to form PbH - , in which Pb is in oxidation state zero. In the energy-minimized structure [H-Pb(η 2 -H 2 )] - , proposed as an intermediate for the process, a H 2 molecule is coordinated with PbH - as a dihapto ligand. The formate adducts of strontium and barium produce monoleptic ions such as [M(0)(η 2 -O 2 CH) 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 η 2 -H 2 complex, which eliminates dihydrogen to form the bismuthide anion (Bi - ).