This chapter discusses a review of the anionic transition-metal hydrides. A large portion of homogeneous catalytic processes of import to the reduction of unsaturated organics, the reduction and homologation of oxycarbon compounds, and the use of carbon monoxide as a reducing agent have as catalysts, catalyst precursors, or mechanistically required intermediates, anionic transition-metal hydride complexes. Encapsulated in a cryptand ligand, the interaction of [K(crypt.222)]+ with an equatorial CO oxygen of -HCr2(CO)10is predictably less perturbing of the anion. Asymmetric hydride bridges are observed for inherently asymmetric dimmers, such as -H[Mo(CO)5] [Mo(CO)4PPh3] in which the MoH distance to the more electron-rich Mo center, Mo(CO)4PPh3, is considerably longer than that to the more electron-poor Mo(CO)5. The X-ray diffraction study revealed a distorted octahedral disposition of ligands about Ru. The crystal structure of the nonsolvated lithium salt of H5W(PMe3)3 disclosed a tetrameric cluster with alternating distances (2.77 and 2.93 A) between Li and W, suggesting the presence of H ligands, which could not be located by X-ray crystallographic methods. Infrared and nuclear magnetic resonance spectroscopies are the primary tools for analyzing the solution structures of transition-metal hydrides. The metalhydrogen stretch should also be useful for the analysis of the electronic character of a particular anion; however, this vibrational frequency is rarely available because of the fact that it is usually a weak and broad absorption or is obscured by other absorptions, such as v(CO) bands. 1987, Academic Press Inc.