Metal ammine complex
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.
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 Werner developed his Nobel Prize-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].H2O 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 analysis.
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+.
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.
Metal ammine complexes find many uses. Cisplatin (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:
- [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−
- A. von Zelewsky "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. ISBN 047195599.
- Atoji, M.; Richardson, J. W.; Rundle, R. E. (1957). "On the Crystal Structures of the Magnus Salts, Pt(NH3)4PtCl4". J. Am. Chem. Soc. 79 (12): 3017–3020. doi:10.1021/ja01569a009.
- "Werner Centennial" George B. Kauffman, Ed. Adv. Chem. Ser., 1967, Volume 62. ISBN 978-0-8412-0063-0
- von Zelewsky, A. "Stereochemistry of Coordination Compounds" John Wiley: Chichester, 1995. ISBN 047195599.
- Alfred Werner "Beitrag zur Konstitution anorganischer Verbindungen" Zeitschrift für anorganische Chemie 1893, Volume 3, pages 267–330.doi:10.1002/zaac.18930030136
- Basolo, F.; Pearson, R. G. "Mechanisms of Inorganic Reactions." John Wiley and Son: New York: 1967. ISBN 0-471-05545-X
- Reinecke, A. "Über Rhodanchromammonium-Verbindungen" Annalen der Chemie und Pharmacie, volume 126, pages 113-118 (1863). doi: 10.1002/jlac.18631260116.
- S. J. Lippard, J. M. Berg “Principles of Bioinorganic Chemistry” University Science Books: Mill Valley, CA; 1994. ISBN 0-935702-73-3.