Among the following,the $\pi$-acid ligand is

Copper sulphate solution reacts with $KCN$ to give

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

$CuSO_4$ solution reacts with excess $KCN$ to give

Given the following reactions:
$Ag^{+} + NH_3 \rightleftharpoons [Ag(NH_3)]^+ ; K_1 = 1.6 \times 10^3$
$[Ag(NH_3)]^+ + NH_3 \rightleftharpoons [Ag(NH_3)_2]^+ ; K_2 = 6.8 \times 10^3$
What is the formation constant for $[Ag(NH_3)_2]^+$?

In the isoelectronic series of metal carbonyls,the $CO$ bond strength is expected to increase in the order: