Talk:Hexaamminecobalt(III) chloride

Bronsted acidity

The complex cation [Co(NH₃)₆]³⁺ is referred to as a "moderately strong" Bronsted acid. Since this is a little surprising, it would be of interest to add its pK_a value as well as a reference. Dirac66 19:58, 21 July 2007 (UTC)

I have tried unsuccesfully to find this number (a reference would be welcome), but the pK_a for the pentammine aquo is 5.8. Maybe I should remove the word "strong".--Smokefoot 21:14, 21 July 2007 (UTC)

The pentammine aquo complex is different as the proton donor is the water ligand:

 [Co(NH3)5(H2O)]3+ + H2O  =  [Co(NH3)5(OH)]2+ +  H3O+. 

The Co³⁺ polarizes H₂O and loosens the O-H bond, reducing the pK_a from 14 in pure water to 5.8 in the complex.

But in the hexammine complex the proton can only come from NH₃, which is a much weaker acid than water. In water solvent the acidity of NH₃ is undetectable, but an estimate would be its acidity in liquid ammonia, where pK_a(NH₃) is about 27 at 298 K (under pressure). The Co³⁺ ion would polarize the NH₃ somewhat, but to produce any detectable acidity in water solvent the pK_a would have to be reduced all the way to 14. I doubt this and so I would remove the whole sentence - unless you can find a specific reference that the hexammine complex (with no water ligand) has detectable acidity in water solvent. Dirac66 23:35, 21 July 2007 (UTC)

Thanks for the suggestion - you are correct about the probable Bronsted site being the water ligand in the pentammine aquo thing. But ammines can be fairly acidic, I just have not seen the data for a few years. FYI, an appropriate comparison for (NH₃)₅CoNH₂-H³⁺ ionization is with NH₃-H⁺, not NH₂-H. Most chemists are surprised by this aspect, but its kind of archane. Your comments will motivate me to dig up a good source, and until then, I will comment out the section that is of concern.--Smokefoot 01:40, 22 July 2007 (UTC)

Properties and structure

This section of the main page needs some references.

Name

In the body text, hexAammine is used, in the title hexammine. Should body text be adapted to match the title or should the title be adapted? Simon de Danser (talk) 11:18, 11 January 2017 (UTC)

The refs are inconsistent (one Inorg. Synth. uses each!). The infobox says the double-"aa" spelling for IUPAC and also gives the double-"aa" as an alternative, which is confusing (surely one form is systematic and "the other" "a" vs "aa" is alternative?). DMacks (talk) 03:31, 21 August 2017 (UTC)

Well sorry. I think I started this monster. It and Tris(ethylenediamine)cobalt(III) chloride have required regular servicing to capture all the possible names. The ammine vs amine is one problem, the hexaam.. vs hexam... is another. The chloride counterions dont help. I am not territorial, yet at least, so if you see issues, there is no problem if you try repairs. I'll take a look in a day or so.--Smokefoot (talk) 02:47, 22 August 2017 (UTC)