Metal ammine complex

In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia (NH₃) ligand. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelt with a single "m". Almost all metal ions bind ammonia as a ligand, but the most prevalent examples of ammine complexes are for Cr(III), Co(III), Ni(II), Cu(II) as well as several platinum group metals.

History

Ammine complexes played a major role in the development of coordination chemistry, specifically determination of the stereochemistry and structure. They are easily prepared, and the metal-nitrogen ratio can be determined by elemental analysis. One of the first ammine complexes to be described was "Magnus' green salt" [Pt(NH₃)₄][PtCl₄]. Relying mainly on the ammine complexes, Alfred Werner developed his Nobel Prize-winning concept of the structure of coordination compounds (see Figure).

Examples

Cobalt(III) and chromium(III)

In the history of coordination chemistry, the ammines of chromium(III) and cobalt(III) were of significance. Both families of amines are relatively inert kinetically, which allows the separation of isomers. For example, tetraamminedichlorochromium(III) chloride, [Cr(NH₃)₄Cl₂]Cl, has two forms - the cis isomer is violet, while the trans isomer is green. The trichloride of the hexaammine (hexamminecobalt(III) chloride, [Co(NH₃)₆]Cl₃)) exists as only a single isomer. "Reinecke's salt" with the formula NH₄[Cr(NCS)₄(NH₃)₂].H₂O was first reported in 1863.