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Copper cyanide forms a complex coordination compound, when dissolved in a solution containing excess ($\ce{CN^-}$) ions.
My question is whether it forms $\ce{[Cu(CN)_4]^{3-}}$ only or both $\ce{[Cu(CN)_3]^{2-}}$ and $\ce{[Cu(CN)_4]^{3-}}$ complexes?
Wikipedia says that both complexes are formed, but I am not sure if its a reliable source.
No, both $\ce{[Cu(CN)3]^2-}$ and $\ce{[Cu(CN)4]^3-}$ species are formed. According to Wikipedia article of copper cyanide:
... but rapidly dissolves in solutions containing $\ce{CN−}$ to form $\ce{[Cu(CN)3]^2-}$ and $\ce{[Cu(CN)4]^3-}$, which exhibit trigonal planar and tetrahedral coordination geometry, respectively. These complexes contrast with those of silver and gold cyanides, which form $\ce{[M(CN)2]−}$ ions in solution.The coordination polymer $\ce{KCu(CN)2}$ contains $\ce{[Cu(CN)2]−}$ units, which link together forming helical anionic chains.
See this paper for more details.
