Hexaamminecobalt(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
CAS Number	10534-89-1 Y
3D model (JSmol)	Interactive image
ChemSpider	140101
ECHA InfoCard	100.030.991
EC Number	234-103-9
PubChem CID	159295
UNII	240056WZHT Y
CompTox Dashboard (EPA)	DTXSID1044358
InChI InChI=1S/3ClH.Co.6H3N/h3*1H;;6*1H3/q;;;+3;;;;;;/p-3 Key: JXBGZYGSWFSYFI-UHFFFAOYSA-K
SMILES [NH3+][Co-3]([NH3+])([NH3+])([NH3+])([NH3+])[NH3+].[Cl-].[Cl-].[Cl-]
Properties
Chemical formula	H18N6Cl3Co
Molar mass	267.48 g/mol
Appearance	yellow or orange crystals
Density	1.71 g/cm3,
Melting point	decomposes
Solubility in water	0.26 M (20 °C) tribromide: 0.04 M (18 °C)
Solubility	soluble in NH3
Structure
Coordination geometry	octahedral
Dipole moment	0 D
Hazards
Occupational safety and health (OHS/OSH):
Main hazards	poison
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, 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). N verify (what is YN ?) Infobox references

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.^[1] 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.^{[citation needed]}

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.^[1] This salt appears to have been first reported by Fremy.^[2]

The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol.^[3] The acetate salt is highly water-soluble to the level of 1.9 M (20 °C), versus 0.26 M for the trichloride.

Uses in the laboratory

[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^[4] or by nuclear magnetic resonance.^[5] 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.^[6]

[Co(NH₃)₆]³⁺ is used to investigate DNA. The cation induces the transition of DNA structure from the classical B-form to the Z-form.^[7]

Related compounds

• Tris(ethylenediamine)cobalt(III) chloride

References

1. Bjerrum, J.; McReynolds, J. P. (1946). "Hexamminecobalt(III) Salts". Inorg. Synth. 2: 216–221. doi:10.1002/9780470132333.ch69.
2. Fremy, M. E. (1852). "Recherches sur le cobalt". Ann. Chim. Phys. 35: 257–312.
3. 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.
4. 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. Bibcode:2003AcCrD..59...67R. doi:10.1107/s0907444902018917. PMID 12499541.
5. 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.
6. McPherson, Alexander (2002). Introduction to Macromolecular Crystallography. John Wiley & Sons. ISBN 0-471-25122-4.
7. Brennant, R. G.; Westhof, E.; Sundaralingam, M. (1986). "Structure of a Z-DNA with Two Different Backbone Chain Conformations. Stabilization of the Decadeoxyoligonucleotide d(CGTACGTACG) by [CO(NH₃)₆]³⁺Binding to the Guanine". Journal of Biomolecular Structure and Dynamics. 3 (4): 649–665. doi:10.1080/07391102.1986.10508453. PMID 3271042.