![Ball-and-stick model of the tetraamminediaquacopper(II) cation,
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[Cu(NH3)4(H2O)2] Tetraamminediaquacopper(II)-3D-balls.png](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a0/Tetraamminediaquacopper%28II%29-3D-balls.png/200px-Tetraamminediaquacopper%28II%29-3D-balls.png)
In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia (NH3) 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]
![Structural representations used by Alfred Werner (right) and Sophus Mads Jorgensen for one isomer of the dichloride salt of the complex
[Pt(NH3)2(pyridine)2]. PtAmineOldvsNewView.png](http://upload.wikimedia.org/wikipedia/commons/thumb/3/34/PtAmineOldvsNewView.png/260px-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 developed his Nobel Prize-winning concept of the structure of coordination compounds (see Figure). [4] [2]
Originally salts of [Co(NH3)6]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. [5]
One of the first ammine complexes to be described was Magnus' green salt, which consists of the platinum tetrammine complex [Pt(NH3)4]2+. [6]
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. [7] [8]
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 [M(NH3)6]n+ have been examined closely by X-ray crystallography. [9]
| 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 [M(NH3)6]n+ where n = 2, 3, and even 4 (M = Pt). [10]
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-[PtCl2(NH3)2], under the name Cisplatin, is an important anticancer drug. Pentamminerhodium chloride ([RhCl(NH3)5]2+) 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. [11] 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. [12]
![Sample of chloropentamminecobalt chloride
[CoCl(NH3)5]Cl2, illustrating the vibrant colors typical of transition metal ammine complexes. CoA5ClCl2.jpg](http://upload.wikimedia.org/wikipedia/commons/thumb/3/39/CoA5ClCl2.jpg/220px-CoA5ClCl2.jpg)
Zinc(II) forms a colorless tetraammine with the formula [Zn(NH3)4]2+. [13] 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(NH3)3]+. [14] Silver gives the diammine complex [Ag(NH3)2]+ with linear coordination geometry. [15] 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 [Au(NH3)2]+. [16]
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 [Co(NH3)6]3+ in aqueous solution and the nonexistence of [Co(H2O)6]3+ (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–NH3 bond is weak, the ammine ligand dissociates and protonation ensues. The behavior is illustrated by the respective non-reaction and reaction with [Co(NH3)6]3+ and [Ni(NH3)6]2+ toward aqueous acids.
The ammine ligands are more acidic than is ammonia (pKa ~ 33). For highly cationic complexes such as [Pt(NH3)6]4+, the conjugate base can be obtained. The deprotonation of cobalt(III) ammine-halide complexes, e.g. [CoCl(NH3)5]2+ 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: [17]
In some ammine complexes, the N–H bond is weak. Thus one tungsten ammine complex evolve hydrogen: [17]
This behavior is relevant to the use of metal-ammine complexes as catalysts for the oxidation of ammonia. [18]
Metal ammine complexes find many uses. Cisplatin (cis-[PtCl2(NH3)2]) is a drug used in treating cancer. [19] 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 [RhCl(NH3)5]Cl2. In some separation schemes, palladium is purified by manipulating equilibria involving [Pd(NH3)4]Cl2, [PdCl2(NH3)2], and [Pt(NH3)4][PtCl4] (Magnus's green salt).
In the processing of cellulose, the copper ammine complex known as Schweizer's reagent ([Cu(NH3)4(H2O)2](OH)2) 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 ([Ag(NH3)2]F) is a topical medicament (drug) used to treat and prevent dental caries (cavities) and relieve dentinal hypersensitivity. [20]
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 complexing agents. Many metal-containing compounds, especially those that include transition metals, are coordination complexes.
Iron(III) chloride describes the inorganic compounds with the formula FeCl3(H2O)x. Also called ferric chloride, these compounds are some of the most important and commonplace compounds of iron. They are available both in anhydrous and in hydrated forms which are both hygroscopic. They feature iron in its +3 oxidation state. The anhydrous derivative is a Lewis acid, while all forms are mild oxidizing agents. It is used as a water cleaner and as an etchant for metals.
Zinc chloride is an inorganic chemical compound with the formula ZnCl2·nH2O, with n ranging from 0 to 4.5, forming hydrates. Zinc chloride, anhydrous and its hydrates, are colorless or white crystalline solids, and are highly soluble in water. Five hydrates of zinc chloride are known, as well as four forms of anhydrous zinc chloride. All forms of zinc chloride are deliquescent. Zinc chloride finds wide application in textile processing, metallurgical fluxes, and chemical synthesis. In a major monograph, zinc chlorides have been described as "one of the important compounds of zinc."
Manganese(II) chloride is the dichloride salt of manganese, MnCl2. This inorganic chemical exists in the anhydrous form, as well as the dihydrate (MnCl2·2H2O) and tetrahydrate (MnCl2·4H2O), with the tetrahydrate being the most common form. Like many Mn(II) species, these salts are pink, with the paleness of the color being characteristic of transition metal complexes with high spin d5 configurations.
Copper(II) chloride, also known as cupric chloride, is an inorganic compound with the chemical formula CuCl2. The monoclinic yellowish-brown anhydrous form slowly absorbs moisture to form the orthorhombic blue-green dihydrate CuCl2·2H2O, with two water molecules of hydration. It is industrially produced for use as a co-catalyst in the Wacker process.
Chromium(III) chloride (also called chromic chloride) is an inorganic chemical compound with the chemical formula CrCl3. It forms several hydrates with the formula CrCl3·nH2O, among which are hydrates where n can be 5 (chromium(III) chloride pentahydrate CrCl3·5H2O) or 6 (chromium(III) chloride hexahydrate CrCl3·6H2O). The anhydrous compound with the formula CrCl3 are violet crystals, while the most common form of the chromium(III) chloride are the dark green crystals of hexahydrate, CrCl3·6H2O. Chromium chlorides find use as catalysts and as precursors to dyes for wool.
Nickel(II) chloride (or just nickel chloride) is the chemical compound NiCl2. The anhydrous salt is yellow, but the more familiar hydrate NiCl2·6H2O is green. Nickel(II) chloride, in various forms, is the most important source of nickel for chemical synthesis. The nickel chlorides are deliquescent, absorbing moisture from the air to form a solution. Nickel salts have been shown to be carcinogenic to the lungs and nasal passages in cases of long-term inhalation exposure.
Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble trihydrated (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.
Platinum(II) chloride is the chemical compound PtCl2. It is an important precursor used in the preparation of other platinum compounds. It exists in two crystalline forms, but the main properties are somewhat similar: dark brown, insoluble in water, diamagnetic, and odorless.
Pentaamine(nitrogen)ruthenium(II) chloride is an inorganic compound with the formula [Ru(NH3)5(N2)]Cl2. It is a nearly white solid, but its solutions are yellow. The cationic complex is of historic significance as the first compound with N2 bound to a metal center. [Ru(NH3)5(N2)]2+ adopts an octahedral structure with C4v symmetry.
Metal halides are compounds between metals and halogens. Some, such as sodium chloride are ionic, while others are covalently bonded. A few metal halides are discrete molecules, such as uranium hexafluoride, but most adopt polymeric structures, such as palladium chloride.
Chloropentamminecobalt chloride is the dichloride salt of the coordination complex [Co(NH3)5Cl]2+. It is a red-violet, diamagnetic, water-soluble salt. The compound has been of academic and historical interest.
trans-Dichlorodiammineplatinum(II) is the trans isomer of the coordination complex with the formula trans-PtCl2(NH3)2, sometimes called transplatin. It is a yellow solid with low solubility in water but good solubility in DMF. The existence of two isomers of PtCl2(NH3)2 led Alfred Werner to propose square planar molecular geometry. It belongs to the molecular symmetry point group D2h.
Hexaamminenickel chloride is the chemical compound with the formula [Ni(NH3)6]Cl2. It is the chloride salt of the metal ammine complex [Ni(NH3)6]2+. The cation features six ammonia (called ammines in coordination chemistry) ligands attached to the nickel(II) ion.
Hexaammineplatinum(IV) chloride is the chemical compound with the formula [Pt(NH3)6]Cl4. It is the chloride salt of the metal ammine complex [Pt(NH3)6]4+. The cation features six ammonia (called ammines in coordination chemistry) ligands attached to the platinum(IV) ion. It is a white, water soluble solid.
Nitrate chlorides are mixed anion compounds that contain both nitrate (NO3−) and chloride (Cl−) ions. Various compounds are known, including amino acid salts, and also complexes from iron group, rare-earth, and actinide metals. Complexes are not usually identified as nitrate chlorides, and would be termed chlorido nitrato complexes.
In organometallic chemistry, transition metal complexes of nitrite describes families of coordination complexes containing one or more nitrite ligands. Although the synthetic derivatives are only of scholarly interest, metal-nitrite complexes occur in several enzymes that participate in the nitrogen cycle.
A transition metal imidazole complex is a coordination complex that has one or more imidazole ligands. Complexes of imidazole itself are of little practical importance. In contrast, imidazole derivatives, especially histidine, are pervasive ligands in biology where they bind metal cofactors.
Cobalt compounds are chemical compounds formed by cobalt with other elements.
Transition metal complexes of thiocyanate describes coordination complexes containing one or more thiocyanate (SCN-) ligands. The topic also includes transition metal complexes of isothiocyanate. These complexes have few applications but played significant role in the development of coordination chemistry.