While studying about copper sulphate, I came across a reaction involving sodium thiosulphate (Hypo) and copper sulphate.

Although I don't know the utility of this reaction, I felt that it was important to understand the structure of $\ce{Na4[Cu6(S2O3)5]}$, which is formed as a the major product of the overall reaction.

The full set of reactions is given as follows:

$$ \begin{align} \ce{CuSO4 + Na2S2O3 &-> CuS2O3 + Na2SO4}\\ \ce{2CuS2O3 + Na2S2O3 &-> Cu2S2O3 + Na2S4O6}\\ \ce{3CuS2O3 + 2Na2S2O3 &-> Na4[Cu6(S2O3)5]} \end{align} $$

What is the structure (3D, not Lewis structure) of $\ce{Na4[Cu6(S2O3)5]}$ (or the ion $\ce{[Cu6(S2O3)5]^{-4}}$)?

Reference: Allen JEE-Mains+Advanced Chemistry, Unit 10, Chapter: Transition Elements:



For knowing these three dimensional complex structures, which are hard to predict theoretically, we have to use X-Ray diffraction or spectroscopic techniques (IR, NMR, or, mass). I don't have any such facilities to verify the complex structure, but I may logically predict a logical (possibly incorrect) structure.

In the complex ion $\ce{[Cu_6(S_2O_3)_5]^4-}$, all the $\ce{Cu}$ is in +1 oxidation state, and thus can form a tetrahedral geometry around it with the help of other $\ce{Cu^+}$ ions and $\ce{S_2O_3^2-}$ ions. On the other hand, thiosulfate can act as both a bidentate and a monodentate ligand.

So, in the predicted structure, all the $\ce{Cu^+}$ are in tetrahedral geometry. Three $\ce{S_2O_3^2-}$ act as bidentate ligands, whereas the other two $\ce{S_2O_3^2-}$ ions act as monodentate ligands.

Note that the donor atom of the thiosulfate anion can be either all $\ce{S}$, all $\ce{O}$, or a combination of $\ce{S}$ and $\ce{O}$. It is indeed hard to predict (because in some complexes, $\ce{Cu}$ binds with $\ce{O}$ regardless of being soft acid to preferably bind with $\ce{S}$ - as in $\ce{CuCl_2.4dmso}$) and thus is best to determine through IR spectroscopy by looking at $\ce{S-S}$ stretching frequency. Thus, the donor atoms may be different from what is shown in the possible structure.

The possible structure is:

enter image description here

  • $\begingroup$ In that case, two $Cu^+$ will be tri-coordinated, which is very unlikely to happen. and , more Importantly, This is a possible Structure. $\endgroup$ – Soumik Das Apr 11 '18 at 13:16
  • 1
    $\begingroup$ Has no spectroscopy or X-RAY diffraction ever been done for this compound? Is it really that uncommon? $\endgroup$ – Abhigyan Chattopadhyay Apr 11 '18 at 16:21
  • $\begingroup$ Spent quite a number of hours spread over days, to end up with no reference of the complex Could someone please refer to any related primary or secondary reference of the complex... $\endgroup$ – Che Mistry Apr 18 at 16:19

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