$AgCl$ is soluble in

Which of the following ligands does not act as a $\pi$-acid ligand?

Which of the following cations will form the most stable complex with the same ligand?

$CuSO_4$ reacts with $KCN$ solution and forms:

Which of the following complexes is the most stable?

When concentrated sulphuric acid is added slowly to a solution of ferrous sulphate containing nitrate ion,a brown coloured ring is formed. The composition of the ring is