Transition metal hydrides

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some so-called hydrides are acidic (e.g., H₂Fe(CO)₄), whereas some others are hydridic, having H⁻-like character (e.g., ZnH₂).

Many transition metals form compounds with hydrogen, called binary hydrides because they only contain two elements. These compounds are invariably insoluble in all solvents. They often exhibit metal-like electrical conductivity. Many are nonstoichiometric compounds. Electropositive metals (Ti, Zr, Hf, Zn) and some other metals form hydrides with the stoichiometry MH or sometimes MH₂ (M = Ti, Zr, Hf, V, Zn). The best studied are the binary hydrides of palladium, which readily forms a limiting monohydride. In fact, hydrogen gas diffuses through Pd windows via the intermediacy of PdH.

Ternary metal hydrides have the formula A_xMH_n, where A⁺ is an alkali or alkaline earth metal cation, e.g. K⁺ and Mg²⁺. A celebrated example is K₂ReH₉, a salt containing two K⁺ ions and the ReH₉²⁻ anion. Other homoleptic metal hydrides include the anions in Mg₂FeH₆ and Mg₂NiH₄. Some of these anionic polyhydrides satisfy the 18-electron rule, many do not. Because of their high lattice energy, these salts are typically not soluble in any solvents, a well known exception being K₂ReH₉.

The most prevalent hydrides of the transition metals are metal complexes that contain a mix of other ligands in addition to hydride. The range of coligands is large. Virtually all of the metals form such derivatives. The main exceptions include the late metals silver, gold, cadmium, and mercury, which form few or unstable complexes with direct M-H bonds. Examples of an industrially useful hydrides are HCo(CO)₄ and HRh(CO)₂(PPh₃)₃, which are catalysts for hydroformylation.

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