Skip to main content
2 of 2
MathJaxified, better title
Jan
  • 68.7k
  • 12
  • 205
  • 389

Why does heating transform the aquacomples of slightly acidified cobalt(II) chloride solution to the chlorido complex?

There is a famous demonstration where, if you mix a $\ce{CoCl2}$-solution with the right amount of $\ce{HCl}$ you will end up with the pink $\ce{[Co(H2O)6]^2+}$ complex but when heating it to boiling temperatures it will change to blue as the $\ce{[CoCl4]^2-}$ complex is formed. This reaction is reversible as the mixture is cooled again. Now literature always describes that this works because that shift towards $\ce{[CoCl4]^2-}$ is endothermic, but why is this the case?

What causes one to be more stable here than the other one but being substituted when heated?

I have some ideas but I can't really compare them:

  • First of all there should actually be an entropic advantage towards the chlorido-side, as 6 water ligands leave, meaning that the exothermic step here is against the natural direction of the entropy(?).

  • Second, perhaps the chlorido ligand is the better leaving group. I know usually $\ce{H2O}$ is a good leaving group in organic molecules but then its usually a $\ce{H2O+}$. So perhaps chloride is the better leaving group here.

  • Maybe there is a kinetic effect, that it is harder to displace 6 ligands than just four ligands

  • Or, as much more water is present it just shifts towards the water side because of the huge excess of water?

  • Chloride is a weaker ligand than water. But on the other hand the iron-fluorido complex is quite stable in water and towards other ligands.

  • So could it be due to the different ligand fields? The splitting in the tetrahedron is much smaller than in the octahedron. Although I don't know how this actually affects the stability.

So those were the ideas I had but I can't really find a common thing among them besides the excess of water which often causes an equlibrium to shift.

By the way $\ce{[Cu(H2O)6]^2+}$ and $\ce{[CuCl4]^2-}$ do the exact same thing.

Does anyone have an idea why this reaction is endothermic?