Hexamminecobalt(III) chloride

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Hexaamminecobalt(III) chloride
chloride
Hexamminecobalt(III)Chloride.jpg
Names
IUPAC name
Hexaamminecobalt(III) chloride
Other names
Cobalt hexammine chloride, hexaamminecobalt(III) chloride
Identifiers
ECHA InfoCard 100.030.991
UNII
Properties
H18N6Cl3Co
Molar mass 267.48 g/mol
Appearance yellow or orange crystals
Density 1.71 g/cm3,
Melting point decomposes
0.26 M (20 °C)
tribromide: 0.04 M (18 °C)
Solubility soluble in NH3
Structure
octahedral
0 D
Hazards
Main hazards poison
R-phrases (outdated) 36/37/38
S-phrases (outdated) none
Related compounds
Other anions
[Co(NH3)6]Br3
[Co(NH3)6](OAc)3
Other cations
[Cr(NH3)6]Cl3
[Ni(NH3)6]Cl2
Related compounds
[Co(H2NCH2CH2NH2)3]Cl3

[Co(NH3)5(H2O)]Cl3
[Co(NH3)5Cl]Cl2

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Hexaamminecobalt(III) chloride is the chemical compound with the formula [Co(NH3)6]Cl3. This coordination compound is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. This salt consists of [Co(NH3)6]3+ cations, each accompanied by three Cl anions. The cation itself is a metal ammine complex with six ammonia molecules as ligands on the cobalt atom.

Originally this compound was described as the luteo (Latin: yellow) complex of cobalt, but this name has been discarded as modern chemistry considers color less important than molecular structure. Other similar complexes also had color names, such as purpureo (Latin: purple) for a cobalt pentammine complex, and praseo (Greek: green) and violeo (Latin: violet) for two isomeric tetrammine complexes. [1]

Properties and structure[edit]

[Co(NH3)6]3+ is diamagnetic, with a low-spin 3d6 octahedral Co(III) center. The cation obeys the 18-electron rule and is considered to be a classic example of an exchange inert metal complex. As a manifestation of its inertness, [Co(NH3)6]Cl3 can be recrystallized unchanged from concentrated hydrochloric acid: the NH3 is so tightly bound to the Co(III) centers that it does not dissociate to allow its protonation. In contrast, labile metal ammine complexes, such as [Ni(NH3)6]Cl2, react rapidly with acids, reflecting the lability of the Ni(II)–NH3 bonds. Upon heating, hexamminecobalt(III) begins to lose some of its ammine ligands, eventually producing a stronger oxidant.

The chloride ions in [Co(NH3)6]Cl3 can be exchanged with a variety of other anions such as nitrate, bromide, and iodide to afford the corresponding [Co(NH3)6]X3 derivative. Such salts are bright yellow and display varying degrees of water solubility.

Preparation[edit]

Since CoCl3 is not available (the Co3+ would oxidise chloride to chlorine gas), [Co(NH3)6]Cl3 is prepared from cobalt(II) chloride using a redox synthesis. The latter is treated with ammonia and ammonium chloride followed by oxidation. Oxidants include hydrogen peroxide or oxygen in the presence of charcoal catalyst.[2] This salt appears to have been first reported by Fremy.[3]

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol.[4] The acetate salt is highly water-soluble to the level of 1.9 M (20 °C), versus 0.26 M for the trichloride.

Uses[edit]

[Co(NH3)6]3+ is a component of some structural biology methods (especially for DNA or RNA, where positive ions stabilize tertiary structure of the phosphate backbone), to help solve their structures by X-ray crystallography[5] or by nuclear magnetic resonance.[6] In the biological system, the counterions would more probably be Mg2+, but the heavy atoms of cobalt (or sometimes iridium, as in PDB file 2GIS) provide anomalous scattering to solve the phase problem and produce an electron-density map of the structure.[7]

[Co(NH3)6]3+ is an unusual example of a water-soluble trivalent metal complex and is of utility for charge-shielding applications such as the stabilization of highly negatively charged complexes e.g. of interactions with and between nucleic acids.

References[edit]

  1. ^ Huheey, James E. (1983). Inorganic Chemistry (3rd ed.). p. 360.
  2. ^ Bjerrum, J.; McReynolds, J. P. (1946). "Hexamminecobalt(III) Salts". Inorg. Synth. 2: 216–221. doi:10.1002/9780470132333.ch69.
  3. ^ Fremy, M. E. (1852). "Recherches sur le cobalt". Ann. Chim. Phys. 35: 257–312.
  4. ^ Lindholm, R. D.; Bause, Daniel E. (1978). "Complexes of Cobalt Containing Ammonia or Ethylene Diamine: Hexaamminecobalt(III) Salts". Inorg. Synth. 18: 67–69. doi:10.1002/9780470132494.ch14.
  5. ^ Ramakrishnan, B.; Sekharudu, C.; Pan, B.; Sundaralingam, M. (2003). "Near-atomic resolution crystal structure of an A-DNA decamer d(CCCGATCGGG): cobalt hexammine interaction with A-DNA". Acta Crystallogr. D59: 67–72. doi:10.1107/s0907444902018917. PMID 12499541.
  6. ^ Rudisser, S.; Tinoco, I., Jr. (2000). "Solution structure of Cobalt(III)hexammine complexed to the GAAA tetraloop, and metal-ion binding to G.A mismatches". J. Mol. Biol. 295: 1211–1232. doi:10.1006/jmbi.1999.3421. PMID 10653698.
  7. ^ McPherson,, Alexander (2002). Introduction to Macromolecular Crystallography. John Wiley & Sons. ISBN 0-471-25122-4.