In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia ligand. "Ammine" is spelled this way for historical reasons;[1] 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.[2]
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. Through studies mainly on the ammine complexes, Alfred Werner developed his Nobel Prize-winning concept of the structure of coordination compounds (see Figure).[3] [2]
Originally salts of [Co(NH<sub>3</sub>)<sub>6</sub>]3+ were described as the luteo (Latin: yellow) complex of cobalt. This name has been discarded as modern chemistry considers color less important than molecular structure. Other metal ammine complexes also were labeled according to their color, such as purpureo (Latin: purple) for a cobalt pentammine complex, and praseo (Greek: green) and violeo (Latin: violet) for two isomeric tetrammine complexes.[4]
One of the first ammine complexes to be described was Magnus' green salt, which consists of the platinum tetrammine complex .[5]
Ammonia is a pure σ-donor, in the middle of the spectrochemical series, and shows intermediate hard–soft behaviour (see also ECW model). Its relative donor strength toward a series of acids, versus other Lewis bases, can be illustrated by C-B plots.[6] [7]
An ammine ligand bound to a metal ion is markedly more acidic than a free ammonia molecule, although deprotonation in aqueous solution is still rare. One example is the reaction of mercury(II) chloride with ammonia (Calomel reaction) where the resulting mercuric amidochloride is highly insoluble.
Ammonia is a Lewis base and a "pure" sigma donor. It is also compact such that steric effects are negligible. These factors simplify interpretation of structural and spectroscopic results.The Co–N distances in complexes have been examined closely by X-ray crystallography.[8]
| M | n+ | M–N distance (Å) | d-electron configuration | comment | |
|---|---|---|---|---|---|
| Co | 3+ | 1.936 | t2g6 eg0 | low-spin trications are small | |
| Co | 2+ | 2.114 | t2g5 eg2 | population of eg orbital and lower positive charge | |
| Ru | 3+ | 2.104 | t2g5 eg0 | low spin trication, but Ru is intrinsically larger than Co | |
| Ru | 2+ | 2.144 | t2g6 eg0 | low spin dication |
Homoleptic poly(ammine) complexes are known for many of the transition metals. Most often, they have the formula where n = 2, 3, and even 4 (M = Pt).[9]
Platinum group metals form diverse ammine complexes. Pentaamine(dinitrogen)ruthenium(II) and the Creutz–Taube complex are well-studied examples of historic significance. The complex cis-, under the name Cisplatin, is an important anticancer drug. Pentamminerhodium chloride is an intermediate in the purification of rhodium from its ores.
The ammines of chromium(III) and cobalt(III) are of historic significance. Both families of ammines are relatively inert kinetically, which allows the separation of isomers.[10] For example, tetraamminedichlorochromium(III) chloride,, has two forms - the cis isomer is violet, while the trans isomer is green. The trichloride of the hexaammine (hexamminecobalt(III) chloride,) exists as only a single isomer. "Reinecke's salt" with the formula was first reported in 1863.[11]
Zinc(II) forms a colorless tetraammine with the formula .[12] Like most zinc complexes, it has a tetrahedral structure. Hexaamminenickel is violet, and the copper(II) complex is deep blue. The latter is characteristic of the presence of copper(II) in qualitative inorganic analysis.
Copper(I) forms only labile complexes with ammonia, including the trigonal planar [Cu(NH<sub>3</sub>)<sub>3</sub>]+.[13] Silver gives the diammine complex [Ag(NH<sub>3</sub>)<sub>2</sub>]+ with linear coordination geometry.[14] It is this complex that forms when otherwise rather insoluble silver chloride dissolves in aqueous ammonia. The same complex is the active ingredient in Tollens' reagent. Gold(I) chloride reacts with ammonia to form .[15]
Since ammonia is a stronger ligand in the spectrochemical series than water, metal ammine complexes are stabilized relative to the corresponding aquo complexes. For similar reasons, metal ammine complexes are less strongly oxidizing than are the corresponding aquo complexes. The latter property is illustrated by the stability of in aqueous solution and the nonexistence of (which would oxidize water).
Once complexed to a metal ion, ammonia is no longer basic. This property is illustrated by the stability of some metal ammine complexes in strong acid solutions. When the M– bond is weak, the ammine ligand dissociates and protonation ensues. The behavior is illustrated by the respective non-reaction and reaction with and toward aqueous acids.
The ammine ligands are more acidic than is ammonia (pKa ~ 33). For highly cationic complexes such as , the conjugate base can be obtained. The deprotonation of cobalt(III) ammine-halide complexes, e.g. labilises the Co–Cl bond, according to the Sn1CB mechanism.
Deprotonation can be combined with oxidation, allowing the conversion of ammine complexes into nitrosyl complexes:
In some ammine complexes, the N–H bond is weak. Thus one tungsten ammine complex evolve hydrogen:[16]
This behavior is relevant to the use of metal-ammine complexes as catalysts for the oxidation of ammonia.[17]
Metal ammine complexes find many uses. Cisplatin (cis-) is a drug used in treating cancer.[18] Many other amine complexes of the platinum group metals have been evaluated for this application.
In the separation of the individual platinum metals from their ore, several schemes rely on the precipitation of . In some separation schemes, palladium is purified by manipulating equilibria involving,, and (Magnus's green salt).
In the processing of cellulose, the copper ammine complex known as Schweizer's reagent is sometimes used to solubilise the polymer. Schweizer's reagent is prepared by treating an aqueous solutions of copper(II) ions with ammonia. Initially, the light blue hydroxide precipitates only to redissolve upon addition of more ammonia:
Silver diammine fluoride is a topical medicament (drug) used to treat and prevent dental caries (cavities) and relieve dentinal hypersensitivity.[19]