Talk:Cobalt(III) hexammine chloride

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[edit] 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 pKa 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 pKa 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 pKa 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 pKa(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 pKa 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)