![Tetraamminediaquacopper(II)-3D-balls](https://upload.wikimedia.org/wikipedia/commons/a/a0/Tetraamminediaquacopper%28II%29-3D-balls.png)
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 complex
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 ...
es containing at least one
ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
()
ligand
In coordination chemistry, a ligand is an ion or molecule (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 electr ...
. "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
![PtAmineOldvsNewView](https://upload.wikimedia.org/wikipedia/commons/3/34/PtAmineOldvsNewView.png)
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
The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...
-winning concept of the structure of coordination compounds (see Figure).
[
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 of ...
, which consists of the platinum tetrammine complex .
Structure and bonding
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
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
.
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
The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six noble, precious metallic elements clustered t ...
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 chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, breast cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, br ...
, is an important anticancer drug. Pentamminerhodium chloride
Pentamminechlororhodium dichloride is the dichloride salt of the coordination complex . It is a yellow, water-soluble solid. The salt is an intermediate in the purification of rhodium from its ores.
As shown by X-ray crystallography, the salt co ...
() is an intermediate in the purification of rhodium from its ores.
File:Carboplatin-skeletal.svg, Carboplatin
Carboplatin, sold under the trade name Paraplatin among others, is a chemotherapy medication used to treat a number of forms of cancer. This includes ovarian cancer, lung cancer, head and neck cancer, brain cancer, and neuroblastoma. It is used b ...
, a widely used anticancer drug.
File:(RhA5Cl)Cl2.png, Pentamminerhodium chloride
Pentamminechlororhodium dichloride is the dichloride salt of the coordination complex . It is a yellow, water-soluble solid. The salt is an intermediate in the purification of rhodium from its ores.
As shown by X-ray crystallography, the salt co ...
, 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 whe ...
.
File:CoA6Cl3.png, 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)6sup>3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination c ...
, the trichloride salt of the hexammine complex . It is famously stable in concentrated hydrochloric acid.
File:Reinecke's salt.png, Reinecke's salt 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
Hexaamminecobalt(III) chloride is the chemical compound with the formula o(NH3)6l3. It is the chloride salt of the coordination complex o(NH3)6sup>3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination c ...
, ) 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
Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may ...
.
Copper(I), silver(I), and gold(I)
Copper(I) forms only labile complexes with ammonia, including the trigonal planar 3)3">u(NH3)3sup>+. Silver gives the diammine complex 3)2">g(NH3)2sup>+ with linear coordination geometry. It is this complex that forms when otherwise rather insoluble silver chloride
Silver chloride is a chemical compound with the chemical formula Ag Cl. This white crystalline solid is well known for its low solubility in water (this behavior being reminiscent of the chlorides of Tl+ and Pb2+). Upon illumination or heating, ...
dissolves in aqueous ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
. 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 nit ...
. 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
Gold(III) chloride, traditionally called auric chloride, is a compou ...
reacts with ammonia to form .
Reactions
Ligand exchange and redox reactions
Since ammonia is a stronger ligand in the spectrochemical series
A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element. For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the l ...
than water, metal ammine complexes are stabilized relative to the corresponding aquo complex
In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...
es. 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 hypertension.
Metal nitrosyl complexes are complex (chemistry), complexes that contain nitric oxide, NO, bonded to a transition me ...
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
Cisplatin is a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, breast cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, br ...
(''cis''-) is a drug used in treating cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
. 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
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wall ...
, the copper ammine complex known as Schweizer's reagent
Schweizer's reagent is the metal ammine complex with the formula u(NH3)4(H2O)2OH)2. This deep-blue compound is used in purifying cellulose.
It is prepared by precipitating copper(II) hydroxide from an aqueous solution of copper sulfate using sod ...
() 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
A medication (also called medicament, medicine, pharmaceutical drug, 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 medical field and rel ...
(drug) used to treat and prevent dental caries
Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...
(cavities) and relieve dentinal hypersensitivity.
See also
* Ligand field theory
Ligand field theory (LFT) describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valen ...
References
{{Coordination complexes
*