Ligand Exchange of hexaaquacobalt(II) ions with hydroxide ions

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Leah.J
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Is this reaction reversible ?
And I know that it's an acid base reaction but should I still say that ligand exchange take place ?
One final thing, the reaction produces a blue ppt but upon standing the ppt becomes pink .. why ?
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Onlinechemtutor
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Hi there,

It is reversed by addition of dilute hydrochloric acid (the blue/pink precipitate dissolves to reform the pink solution).

It is not ligand exchange. What happens is that the hydroxide ions deprotonate water ligands to change them to hydroxide ligands. Although the effect is the same as ligand exchange it is purely an acid-base reaction. You will be penalised in the exam if you describe it as ligand exchange.

I don't know why it becomes pink on standing, and I think opinion is still divided on this at the higher echelons of chemical research. Needless to say, you are not expected to explain this observation.


Hope this helps!
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charco
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(Original post by Leah.J)
Is this reaction reversible ?
And I know that it's an acid base reaction but should I still say that ligand exchange take place ?
One final thing, the reaction produces a blue ppt but upon standing the ppt becomes pink .. why ?
It becomes grey/pink as the cobalt(II) gets oxidised by the air to cobalt(III)
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Onlinechemtutor
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I don't think that oxidation is the reason for this colour change. Cobalt(III) is usually green, yellow or brown depending upon what other ions or ligands are present. As I said in my reply, I don't think that the reason for the colour change of cobalt(II) hydroxide from blue to pink has yet been fully established. Some resources suggest that it might be caused by a change in coordination number occurring within the mixture.


(Original post by charco)
It becomes grey/pink as the cobalt(II) gets oxidised by the air to cobalt(III)
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