The formation, thermochemical properties, and reactivity of gas-phase pentacoordinate silicon hydride anions are described. These ions are produced as the major products of reactions between H- and alkylsilanes under flowing afterglow conditions at room temperature. Substituted silicon hydride ions are formed by addition of nucleophilic anions such as F-, alkoxides, nitrile anions, and CF3- to SiH4 and to primary, secondary, or tertiary alkylsilanes. The parent ion of the series, SiH5-, is formed by hydride transfer from alkyl silicon hydride ions to SiH4. Pentacoordinate silicon hydride ions are shown to be reactive hydride reducing agents, transferring H- to a wide variety of organic, inorganic, and transition-metal organometallic species. Silicon hydride ions undergo sequential hydride-deuteride exchange reactions with SiD4 by a mechanism analogous to protonic H/D exchange in gas-phase carbanions. Reactions with Bronsted acids lead to protolytic cleavage of an Si-H bond in the anion and formation of H-2 Depending upon the structure and acidity of the reactant acid, these reactions produce the corresponding silyl anion and both the free and silicon-complexed conjugate base anion. Reactions between alkylhydridosiliconate ions and neutral silanes occur by hydride transfer, protolytic cleavage and alkyl group transfer with accompanying H-2 loss. The observation of "self-cleavage" of an alkylhydridosiliconate ion by its alkylsilane precursor to produce the corresponding silyl anion and H-2 indicates that simple alkylhydridosiliconates are metastable with respect to H-2 loss. Reactions between CO2 and monodeuterated alkylhydridosiliconate ions formed by addition of D- to alkylsilanes produce statistical yield ratios of DCO2- and HCO2-, indicating complete scrambling of hydrogen and deuterium ligands in the siliconate ion, and a negligible isotope effect for the exothermic transfer of H- to CO2. The hydride affinity (HA) ordering of alkylsilanes is determined from bracketing methods and equilibrium measurements to be HA(Et(3)SiH) less than or equal to HA(Et(2)SiH(2)) less than or equal to HA(n-C5H11SiH3) less than or equal to HA(SiH4), where the differences among all the silanes are less than 1-2 kcal/mol. The absolute hydride affinities for SiH4 and the alkylsilanes are estimated to be 19-20 kcal/mol. Collisional activation of SiH5- and alkylhydridosiliconate ions results in inefficient fragmentation by H-2 loss, with estimated threshold energies of about 0.6 eV. CID of Et(3)SiHD(-) occurs by loss of both HD and H-2, indicating that the alkyl hydrogens are partially involved in these dissociations.