Periodic Trends Manifested through Gas-Phase Generation of Anions Such as [AlH], [GaH], [InH], [SrH], [BaH], [Ba(0)(η-OCH)], [Pb(0)H], [Bi(I)H], and Bi from Formates.

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