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 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(eta-OCOH)(4)(-) and Ga(eta-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 TI) 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 H-2 to form PbH-, in which Pb is in oxidation state zero. In the energy-minimized structure [H-Pb(eta(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)(eta(2)-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 eta(2)-H-2 complex, which eliminates dihydrogen to form the bismuthide anion (Bi-).