In Huheey's Inorganic Chemistry it is written that hexacyanidocobaltate(II) is uncommon but pentacyanidoaquacobaltate(II) is common. Something is mentioned about Jahn-Teller distortion, but I don't understand how it applies here.

  • $\begingroup$ We use it in the lab all the time, I don't think it's uncommon. $\endgroup$ – Ndrina Limani Apr 27 '15 at 18:10
  • $\begingroup$ that is artificial. i think huheey said not common in nature $\endgroup$ – Punarbasu Roy Apr 28 '15 at 10:00

In Inorganic Syntheses (1946), a synthesis of $\ce{K3[Co(CN)6]}$ is described, which boils down to the following three equations:[1]

$$\begin{align}\ce{CoCl2 (aq) + 2 KCN (aq) &-> Co(CN)2 v + 2 KCl (aq)}\tag{1}\\ \ce{Co(CN)2 + 4 KCN (aq) &-> K4[Co(CN)6] (aq)}\tag{2}\\ \ce{2 K4[Co(CN)6] (aq) + H2O &-> 2 K3[Co(CN)6] (aq) + 2 KOH (aq) + H2}\tag{3}\end{align}$$

The procedure requires precipitation of the cobalt(II) cyanide by carefully adding a $\ce{KCN}$ solution to a cobalt(II) solution. This precipitate is filtrated and washed, then redissolved in excess cyanide solution giving a deep-red solution. This in turn is heated to reflux for some 15 minutes, whereupon the colour changes to yellow indicating the formation of the hexacyanidocobaltate(III).[1]

I personally doubt equation $(3)$. Instead, I would assume that the oxidation of hexacyanidocobaltate(II) to the corresponding cobaltate(III) proceeds unter oxygen consumption, not hydrogen liberation. That would give equation $(\text{3'})$:

$$\ce{4 K4[Co(CN)6] + O2 + 2 H2O -> 4 K3[Co(CN)6] + 4 KOH}\tag{3'}$$

In any case, however, the presence of $\ce{[Co(CN)6]^4-}$ or hexacyanidocobaltate(II) as a stable intermediate in solution suggests to me that your premise is false and thus the book you cite from incorrect.


J. H. Bigelow and J. C. Bailar (1946) Potassium Hexacyanocobaltate(III), in: Inorganic Syntheses, Volume 2 (ed W. C. Fernelius), John Wiley & Sons, Inc., Hoboken, NJ, USA. DOI: 10.1002/9780470132333.ch72.


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