What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when H2S(g) is passed through this solution?

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.