Hexamminecobalt(III) chloride

Hexaamminecobalt(III) chloride is the chemical compound with the formula [Co(NH3)6]Cl3. It is the chloride salt of the coordination complex [Co(NH3)6]3+, which is considered an archetypal "Werner complex", named after the pioneer of coordination chemistry, Alfred Werner. The cation itself is a metal ammine complex with six ammonia ligands attached to the cobalt(III) ion.

Hexaamminecobalt(III) chloride
Names
IUPAC name
Hexaamminecobalt(III) chloride
Other names
Cobalt hexammine chloride, hexaamminecobalt(III) chloride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.030.991
EC Number
  • 234-103-9
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
GHS pictograms
GHS Signal word Warning
H315, H319, H335
P261, P264, P271, P280, P302+352, P304+340, P305+351+338, P312, P321, P332+313, P337+313, P362, P403+233, P405, P501
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

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 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

[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, iodide, sulfamate to afford the corresponding [Co(NH3)6]X3 derivative. Such salts are orange or bright yellow and display varying degrees of water solubility. The chloride ion can be also exchanged with more complex anions such as the hexathiocyanatochromate(III), yielding a pink compound with formula [Co(NH3)6][Cr(SCN)6], or the ferricyanide ion.

Preparation

[Co(NH3)6]Cl3 is prepared by treating cobalt(II) chloride 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

[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: 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, such as interactions with and between nucleic acids.

References
  1. Huheey, James E. (1983). Inorganic Chemistry (3rd ed.). p. 360.
  2. Bjerrum, J.; McReynolds, J. P. (1946). Hexamminecobalt(III) Salts. Inorg. Synth. Inorganic Syntheses. 2. pp. 216–221. doi:10.1002/9780470132333.ch69. ISBN 9780470132333.
  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. Inorganic Syntheses. 18. pp. 67–69. doi:10.1002/9780470132494.ch14. ISBN 9780470132494.
  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 (Pt 1): 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 (5): 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.
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