Ammine
Encyclopedia
In coordination chemistry, metal ammine complexes are metal complexes containing at least one ammonia
(NH3) ligand
. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelled 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.
developed his Nobel Prize
-winning concept of the structure of coordination compounds (see Figure).
was first reported in 1863.
.
Because of the formation of the diamminesilver ion ([Ag(NH3)2]+), the otherwise insoluble silver chloride
dissolve in aqueous ammonia
. Such solutions are used in Tollen's reagent.
.
(PtCl2(NH3)2) is a coordination compound containing two chloro and two ammine ligands. This 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 platinum metals, 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[PdCl4].
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:
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3) ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
. "Ammine" is spelled this way due to historical reasons; in contrast, alkyl or aryl bearing ligands are spelled 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.
History
Ammine complexes played a major role in the development of coordination chemistry, specifically determinaton of the stereochemistry and structure. They are easily prepared, and the metal-nitrogen ratio can be determined by elemental analysis. One of the first ammine complexes to be described was "Magnus' green salt" [Pt(NH3)4][PtCl4]. Relying mainly on the ammine complexes, Alfred WernerAlfred Werner
Alfred Werner 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 transition metal complexes. Werner developed the basis for modern coordination chemistry...
developed his Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
-winning concept of the structure of coordination compounds (see Figure).
Cobalt(III) and chromium(III)
In the history of coordination chemistry, the ammines of chromium(III) and cobalt(III) were of significance. Both families of amines are relatively inert kinetically, which allows the separation of isomers. 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].H2OWater of crystallization
In crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...
was first reported in 1863.
Copper(II) and silver(I)
Copper(II) form deep blue ammine complexes, which is characteristic of the presence of copper(II) in qualitative inorganic analysisQualitative inorganic analysis
Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, so that materials in other forms may need to be brought into this state before using standard...
.
Because of the formation of the diamminesilver ion ([Ag(NH3)2]+), the otherwise insoluble silver chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
dissolve in aqueous ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
. Such solutions are used in Tollen's reagent.
Ligand exchange and redox reactions
Since ammonia is a more basic ligand than is water, metal ammine complexes are stabilised relative to the corresponding aquo complexes. For similar reasons, metal ammine complexes are more easily oxidised than are the corresponding aquo complexes. The latter property is illustrated by the stability of [Co(NH3)6]3+ and the nonexistence of [Co(H2O)6]3+.Acid-base reactions
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 mechanismSn1CB mechanism
The SN1CB mechanism describes the pathway by which many metal amine complexes undergo substitution, that is ligand exchange. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base to give the corresponding polyamine metal hydroxide:The rate law for the reaction...
.
Applications
Metal ammine complexes find many uses. CisplatinCisplatin
Cisplatin, cisplatinum, or cis-diamminedichloroplatinum is a chemotherapy drug. It is used to treat various types of cancers, including sarcomas, some carcinomas , lymphomas, and germ cell tumors...
(PtCl2(NH3)2) is a coordination compound containing two chloro and two ammine ligands. This a drug used in treating cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
. Many other amine complexes of the platinum group metals have been evaluated for this application.
In the separation of the platinum metals, 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[PdCl4].
In the processing of cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, the copper ammine complex known as Schweizer's reagent
Schweizer's reagent
Schweizer's reagent is the chemical complex tetraamminediaquacopper dihydroxide, [Cu42]2. It is prepared by precipitating copper hydroxide from an aqueous solution of copper sulfate using sodium hydroxide or ammonia, then dissolving the precipitate in a solution of ammonia.When the entire amount of...
([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:
- [Cu(H2O)6]2+ + 2 OH− → Cu(OH)2 + 6 H2O
- Cu(OH)2 + 4 NH3 + 2 H2O → [Cu(NH3)4(H2O)2]2+ + 2 OH−