Which of the following ions forms the most stable complex compound?

Cold ferrous sulphate solution on absorption of $NO$ develops brown colour due to the formation of

Which of the following is the correct increasing order of ligand field strength according to the spectrochemical series?

Which complex is most stable? Where $k_d =$ Dissociation constant.

The overall stability constant of the complex ion $[Cu(NH_3)_4]^{2+}$ is $2.1 \times 10^{13}$. The overall dissociation constant is $y \times 10^{-14}$. Then $y$ is ....... .(Nearest integer)

The cation that does not form an amine complex with excess of ammonia is