CuSO4 + KCN ⇒ K2[Cu(CN)4] + K2SO4
[Cu(H2O)4]2+ + 4CN- ⇒ [Cu(CN)4]2- + 4H2O
The coordination entity formed is K2[Cu(CN)4] .
IUPAC name of the coordination entity is potassium tetracyanocuprate(II). It is a very stable complex. The copper atom present inside the coordination sphere does not separate out to form copper ions and cyanide ions due to strong bond between them.It does not ionize to give Cu2+ ions and hence on adding H2S , since there are no copper ions present so no precipitate of copper sulfide is formed.
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