ammine group
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In
coordination chemistry A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
, metal ammine complexes are metal complexes containing at least one ammonia () 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.A. von Zelewsky "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. .


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. Through studies mainly on the ammine complexes,
Alfred Werner Alfred Werner (12 December 1866 – 15 November 1919) was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of ...
developed his Nobel Prize-winning concept of the structure of coordination compounds (see Figure). One of the first ammine complexes to be described was Magnus' green salt, which consists of the platinum tetrammine complex .


Structure and bonding

Ammonia is a
Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...
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.


Examples

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).


Platinum group metals

Platinum group metals form diverse ammine complexes. Pentaamine(dinitrogen)ruthenium(II) and the
Creutz–Taube complex The Creutz–Taube ion is the metal complex with the formula 5+. This cationic species has been heavily studied in an effort to understand the intimate details of inner sphere electron transfer, that is, how electrons move from one metal complex t ...
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. File:Carboplatin-skeletal.svg, Carboplatin, a widely used anticancer drug. File:(RhA5Cl)Cl2.png, Pentamminerhodium chloride, the dichloride salt of a pentammine halide complex. File:RuA5N2.png, Pentaamine(dinitrogen)ruthenium(II), the first
metal dinitrogen complex Transition metal dinitrogen complexes are coordination compounds that contain transition metals as ion centers the dinitrogen molecules (N2) as ligands. Historical background Transition metal complexes of N2 have been studied since 1965 when ...
. File:CoA6Cl3.png, Hexamminecobalt(III) chloride, the trichloride salt of the hexammine complex . It is famously stable in concentrated hydrochloric acid. File:Reinecke's salt.png,
Reinecke's salt Reinecke's salt is a chemical compound with the formula NH4 r(NCS)4(NH3)2· H2O. The dark-red crystalline compound is soluble in boiling water, acetone, and ethanol. The chromium atom is surrounded by six nitrogen atoms in an octahedral geometry. ...
features a very stable anionic diamine complex of Cr(III), which is used as a
counteranion image:Polystyrolsulfonat.svg, 160px, Polystyrene sulfonate, a cation-exchange resin, is typically supplied with as the counterion. In chemistry, a counterion (sometimes written as "counter ion", pronounced as such) is the ion that accompanies an ...
.


Cobalt(III) and chromium(III)

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. 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 Reinecke's salt is a chemical compound with the formula NH4 r(NCS)4(NH3)2· H2O. The dark-red crystalline compound is soluble in boiling water, acetone, and ethanol. The chromium atom is surrounded by six nitrogen atoms in an octahedral geometry. ...
" with the formula was first reported in 1863.


Nickel(II), zinc(II), copper(II)

Zinc(II) forms a colorless tetraammine with the formula . 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), silver(I), and gold(I)

Copper(I) forms only labile complexes with ammonia, including the trigonal planar u(NH3)3sup>+. Silver gives the diammine complex g(NH3)2sup>+ with linear coordination geometry. 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 Gold(I) chloride is a compound of gold and chlorine with the chemical formula AuCl. Preparation Gold(I) chloride is prepared by thermal decomposition of gold(III) chloride. Reactions Although there is a region of stability at higher temperatures ...
reacts with ammonia to form .


Reactions


Ligand exchange and redox reactions

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).


Acid-base reactions

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 (p''K''a ~ 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.


Oxidation of ammonia

Deprotonation can be combined with oxidation, allowing the conversion of ammine complexes into
nitrosyl complex Sodium nitroprusside, a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat complexes that contain nitric oxide">hypertension. Metal nitrosyl complexes are complex (chemistry)">complexes that contain nitri ...
es: :


H-atom transfer

In some ammine complexes, the N–H bond is weak. Thus one tungsten ammine complex evolve hydrogen: : This behavior is relevant to the use of ammonia as a hydrogen source.


Applications

Metal ammine complexes find many uses. Cisplatin (''cis''-) is a drug used in treating cancer. 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 . 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 Silver diammine fluoride (SDF), also known as silver diamine fluoride in most of the dental literature, (although this is a chemical misnomer) is a topical medication used to treat and prevent dental caries (tooth decay) and relieve dentinal hype ...
() is a topical medicament (drug) used to treat and prevent dental caries (cavities) and relieve dentinal hypersensitivity.


See also

* Ligand field theory


References

{{Coordination complexes *