Metal amides
Metal amides are a class of coordination compounds composed of a metal center with amide ligands of the form NR2. Amide ligands have two electron pairs available for bonding. In principle, they can be terminal or bridging. In these two examples, the dimethylamido ligands are both bridging and terminal:
Tris(dimethylamino)aluminium dimer
Tris(dimethylamino)gallium dimer
In practice, bulky amide ligands have a lesser tendency to bridge. Amide ligands may participate in metal-ligand π-bonding giving a complex with the metal center being co-planar with the nitrogen and substituents. Metal bis(trimethylsilyl)amides form a significant subcategory of metal amide compounds. These compounds tend to be discrete and soluble in organic solvents.
Lithium amides are the most important amides, as they are readily prepared from n-butyllithium and the appropriate amine, and they are more stable and soluble than the other alkali metal analogs. Potassium amides are prepared by transmetallation of lithium amides with potassium t-butoxide (see also Schlosser base) or by reaction of the amine with potassium, potassium hydride, n-butylpotassium, or benzylpotassium.
The alkali metal amides, MNH2 (M = Li, Na, K) are commercially available. Sodium amide (also known as sodamide) is synthesized from sodium metal and ammonia with ferric nitrate catalyst. The sodium compound is white, but the presence of metallic iron turns the commercial material gray.
Lithium diisopropylamide is a popular non-nucleophilic base used in organic synthesis. Unlike many other bases, the steric bulk prevents this base from acting as a nucleophile. It is commercially available, usually as a solution in hexane. It may be readily prepared from n-butyllithium and diisopropylamine.
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