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A complex salt is a salt where the metal and ligands do not dissociate in water.

For eg : $\space\ce{K_4[Fe(CN)_6]}$ when put into $\space\ce{H_2O}$ dissociates as : $\space\ce{4K^+} + \ce{[Fe(CN)_6]^{4-}}$

where $\ce{[Fe(CN)_6]^{4-}}$ is a complex ion which cannot be further dissociated.

So, is there any way to dissociate them, either by heating or by adding something? Or will they dissociate in some other solvent? Or is there no way to dissociate them?

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  • $\begingroup$ Generally every cation/central atom in water is solvated and usually complexed $\endgroup$ – Mithoron Feb 19 '15 at 14:50
  • $\begingroup$ Complex ions are in chemical equilibrium with their dissociated forms, and this equilibrium can be shifted just as any other equilibrium. In your example, the complex form is very stable compared to the dissociated forms, but still, e.g. adding strong acid (DO NOT TRY IT!) one can free some HCN. $\endgroup$ – Greg Apr 18 '18 at 14:56
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Let's consider the formation constant of this ion complex at $25^\circ\mathrm{C}$: $$\ce{Fe^{2+} + 6CN- <=> [Fe(CN)6]^4-}\qquad K_\mathrm{f}=10^{+24}$$ This means that, the equilibrium is highly displaced towards the formation of the complex ion. The ion complex is very stable.

So, if we want to displace this equilibrium towards the dissociation of the complex ion, we have to add an adequate quantities of a metallic ion or a ligand that form with $\ce{CN^{-}}$ or $\ce{Fe^{2+}}$, respectively more stable complexes.

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