Complex metal hydrides are salts wherein the anions contain hydrides. In the older chemical literature as well as contemporary materials science textbooks, a "metal hydride" is assumed to be nonmolecular, i.e. three-dimensional lattices of atomic ions. In such systems, hydrides are often interstitial and nonstoichiometric, and the bonding between the metal and hydrogen atoms is significantly ionic. In contrast, complex metal hydrides typically contain more than one type of metal or metalloid and are either soluble or react with water. They exhibit ionic bonding between a positive metal ion with molecular anions containing the hydride. In such materials the hydrogen is bonded with significant covalent character to the second metal or metalloid atoms.[1]
In general, complex metal hydrides have the formula MxM'yHn, where M is an alkali metal cation or cation complex and M' is a metal or metalloid. Well known examples feature group 13 elements, especially boron and aluminium including sodium aluminium hydride, NaAlH4 (also known as sodium alanate), and lithium borohydride, (LiBH4). Complex metal hydrides are often soluble in etherialsolvents. Other complex metal hydrides are numerous. Illustrative examples include the salts [MgBr(THF)2]4FeH6 and K2ReH9.[1]
Applications
Complex metal hydrides of boron and aluminium are widely used in organic synthesis for the reduction of carbon-oxygen and carbon-nitrogen multiple bonds.[1] These materials are also of topical interest because of their potential as hydrogen storage materials.[2] A material such as lithium borohydride is capable of storing hydrogen with greater hydrogen density than liquefied methane (and well above the density of liquid hydrogen).[3] Complex metal hydrides often suffer from instability toward water.
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