Amongst the following,the most stable complex is . . . . . . .

When $Co(II)$ chloride is dissolved in concentrated $HCl$,a blue solution is obtained. Upon dilution with water,the colour changes to pink because:

Select the correct $M-C$ bond order.

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 $H_{2}S_{(g)}$ is passed through this solution?

Consider the following complexes:
$(a) K_2PtCl_6$
$(b) PtCl_4 \cdot 2NH_3$
$(c) PtCl_4 \cdot 3NH_3$
$(d) PtCl_4 \cdot 5NH_3$
Their electrical conductances in aqueous solutions are:

Among $[Fe(H_2O)_6]^{3+}$,$[Fe(CN)_6]^{3-}$,$[Fe(C_2O_4)_3]^{3-}$,and $[FeCl_6]^{3-}$,which complex is the most stable?