Hexaamminecobalt(III) chloride
{{chembox
| Verifiedfields = changed
| verifiedrevid = 428254608
| Name = Hexaamminecobalt(III) chloride
| ImageFile = CoA6Cl3.svg
| ImageName = chloride
| ImageFile1 = Hexamminecobalt(III)Chloride.jpg
| ImageName1 =
| IUPACName = Hexaamminecobalt(III) chloride
| OtherNames = Cobalt hexammine chloride, hexaamminecobalt(III) chloride
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|correct|??}}
| CASNo = 10534-89-1
| PubChem = 159295
| ChemSpiderID = 140101
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = 240056WZHT
| EINECS = 234-103-9
| StdInChI=1S/3ClH.Co.6H3N/h3*1H;;6*1H3/q;;;+3;;;;;;/p-3
| StdInChIKey = JXBGZYGSWFSYFI-UHFFFAOYSA-K
| SMILES = [NH3+][Co-3]([NH3+])([NH3+])([NH3+])([NH3+])[NH3+].[Cl-].[Cl-].[Cl-]
| RTECS =
}}
| Section2 = {{Chembox Properties
| Formula = H18N6Cl3Co
| MolarMass = 267.48 g/mol
| Appearance = yellow or orange crystals
| Density = 1.71 g/cm3,
| Solubility = 0.26 M (20 °C)
tribromide: 0.04 M (18 °C)
| SolubleOther = soluble in NH3
| MeltingPt = decomposes
| pKa =
}}
| Section3 = {{Chembox Structure
| Coordination = octahedral
| CrystalStruct =
| Dipole = 0 D
}}
| Section7 = {{Chembox Hazards
| ExternalSDS =
| MainHazards = poison
| GHSPictograms = {{GHS07}}
| GHSSignalWord = Warning
| HPhrases = {{H-phrases|315|319|335}}
| PPhrases = {{P-phrases|261|264|271|280|302+352|304+340|305+351+338|312|321|332+313|337+313|362|403+233|405|501}}
}}
| Section8 = {{Chembox Related
| OtherAnions = [Co(NH3)6]Br3
[Co(NH3)6](OAc)3
| OtherCations = [Cr(NH3)6]Cl3
[Ni(NH3)6]Cl2
| OtherFunction_label = compounds
| OtherFunction = [Co(H2NCH2CH2NH2)3]Cl3
[Co(NH3)5(H2O)]Cl3
[Co(NH3)5Cl]Cl2
}}
}}
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.
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.{{cn|date=September 2023}}
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.{{cite journal | last1= Bjerrum |first1=J. |last2=McReynolds |first2=J. P. | title = Hexamminecobalt(III) Salts | journal = Inorg. Synth. | year = 1946 | volume = 2 | pages = 216–221 | doi = 10.1002/9780470132333.ch69}} This salt appears to have been first reported by Fremy.{{cite journal
| first= M. E. |last=Fremy
| journal = Ann. Chim. Phys.
| year = 1852
| volume = 35
| pages = 257–312
| url = http://gallica.bnf.fr/ark:/12148/bpt6k34776q/f255.table
| title = Recherches sur le cobalt}}
The acetate salt can be prepared by aerobic oxidation of cobalt(II) acetate, ammonium acetate, and ammonia in methanol.{{cite journal | last= Lindholm |first=R. D. | title = Complexes of Cobalt Containing Ammonia or Ethylene Diamine: Hexaamminecobalt(III) Salts | journal = Inorg. Synth. | year = 1978 | volume = 18 | pages = 67–69 | doi = 10.1002/9780470132494.ch14 | last2 = Bause | first2 = Daniel E.}} 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{{cite journal | last1= Ramakrishnan |first1=B. |last2=Sekharudu |first2=C.|last3=Pan |first3=B. |last4=Sundaralingam |first4=M. | title = Near-atomic resolution crystal structure of an A-DNA decamer d(CCCGATCGGG): cobalt hexammine interaction with A-DNA | journal = Acta Crystallogr. | year = 2003 | volume = D59 |issue=Pt 1 | pages = 67–72 | pmid = 12499541 | doi = 10.1107/s0907444902018917|bibcode=2003AcCrD..59...67R }} or by nuclear magnetic resonance.{{cite journal | last1= Rudisser |first1=S. |last2=Tinoco |first2=I. Jr. | title = Solution structure of Cobalt(III)hexammine complexed to the GAAA tetraloop, and metal-ion binding to G.A mismatches. | journal = J. Mol. Biol. | year = 2000 | volume = 295 |issue=5 | pages = 1211–1232 | pmid = 10653698 | doi=10.1006/jmbi.1999.3421}} 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.{{cite book |last= McPherson |first=Alexander|title= Introduction to Macromolecular Crystallography|publisher= John Wiley & Sons|year= 2002|isbn= 0-471-25122-4}}
[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.{{cite journal|doi=10.1080/07391102.1986.10508453 |title=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 |date=1986 |last1=Brennant |first1=R. G. |last2=Westhof |first2=E. |last3=Sundaralingam |first3=M. |journal=Journal of Biomolecular Structure and Dynamics |volume=3 |issue=4 |pages=649–665 |pmid=3271042 }}