The active substance in Nessler's reagent used for the detection of ammonia is ..........

$A$ solution containing $2.675 \ g$ of $CoCl_3 \cdot 6NH_3$ (molar mass $= 267.5 \ g \ mol^{-1}$) is passed through a cation exchanger. The chloride ions obtained in the solution are treated with excess $AgNO_3$ to give $4.78 \ g$ of $AgCl$ (molar mass $= 143.5 \ g \ mol^{-1}$). The formula of the complex is $(Ag = 108 \ u)$:

$AgCl$ dissolves in ammonia solution giving

Copper sulphate solution reacts with $KCN$ to give

Which among the following cations forms the most stable complex with the same ligand?

Which of the following will not give a precipitate with $AgNO_3$?