Ammine
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coordination chemistry A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...
, metal ammine complexes are
metal complex A coordination complex is a chemical compound consisting 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 ...
es containing at least one
ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
()
ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
. "Ammine" is spelled this way for 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 ...
developed his
Nobel Prize The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
-winning concept of the structure of coordination compounds (see Figure). Originally salts of o(NH3)6sup>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. One of the first ammine complexes to be described was
Magnus' green salt Magnus's green salt is the inorganic compound with the formula t(NH3)4PtCl4]. This salt is named after Heinrich Gustav Magnus, who, in the early 1830s, first reported the compound. The compound is a linear chain compound, consisting of a chain o ...
, which consists of the platinum tetrammine complex .


Structure and bonding

Ammonia is a pure σ-donor, in the middle of the spectrochemical series, and shows intermediate hard–soft behaviour (see also ECW model ). Ammonia is classified as an L ligand in the Covalent bond classification method. In the usual electron counting method, it is a two-electron ligand. Ammonia 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 repeatedly by
X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...
.


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 are well-studied examples of historic significance. The complex ''cis''-, under the name
Cisplatin Cisplatin is a chemical compound with chemical formula, formula ''cis''-. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, c ...
, 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. File:CoA6Cl3.svg,
Hexamminecobalt(III) chloride Hexaamminecobalt(III) chloride is the chemical compound with the formula o(NH3)6l3. It is the chloride salt of the coordination complex o(NH3)6+, which is considered an archetypal "Werner complex", named after the pioneer of coordination ...
, the trichloride salt of the hexammine complex . It is famously stable in concentrated
hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
. File:Reinecke's salt.png, Reinecke's salt features a very stable anionic diamine complex of Cr(III), which is used as a counteranion.


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 Hexaamminecobalt(III) chloride is the chemical compound with the formula o(NH3)6l3. It is the chloride salt of the coordination complex o(NH3)6+, which is considered an archetypal "Werner complex", named after the pioneer of coordination ...
, ) exists as only a single isomer. " Reinecke's salt" 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 Silver chloride is an inorganic chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts ...
dissolves in aqueous
ammonia Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
. The same complex is the active ingredient in
Tollens' reagent Tollens' reagent (chemical formula Ag(NH3)2OH) is a chemical reagent used to distinguish between aldehydes and ketones along with some alpha-hydroxy ketones which can tautomerize into aldehydes. The reagent consists of a solution of silver n ...
. Gold(I) chloride 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. Related is the reaction of mercury(II) chloride with ammonia (Calomel reaction), where the resulting mercuric amidochloride is highly insoluble. : 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 complexes: :


H-atom transfer

In some ammine complexes, the N–H bond is weak. Thus one molybdenum ammine complex evolve hydrogen: : This behavior is relevant to the use of metal-ammine complexes as catalysts for the oxidation of ammonia.


Applications

Metal ammine complexes find many uses.
Cisplatin Cisplatin is a chemical compound with chemical formula, formula ''cis''-. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, c ...
(''cis''-) is a drug used in treating
cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...
. 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 Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
, 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 Medication (also called medicament, medicine, pharmaceutical drug, medicinal product, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy ( pharmacotherapy) is an important part of the ...
(drug) used to treat and prevent
dental caries Tooth decay, also known as caries,The word 'caries' is a mass noun, and is not a plural of 'carie'.'' is the breakdown of teeth due to acids produced by bacteria. The resulting cavities may be a number of different colors, from yellow to black ...
(cavities) and relieve dentinal hypersensitivity.


See also

* Ligand field theory


References

{{Coordination complexes *