$PCl_5$ is well known,but $NCl_5$ is not. Why?

In group $15$ of the periodic table,nitrogen forms only a trihalide,but other elements form pentahalides also. The reason is:

On heating lead $(II)$ nitrate gives a brown gas $'A'$. The gas $'A'$ on cooling changes to a colourless solid $'B'$. Solid $'B'$ on heating with $NO$ changes to a blue solid $'C'$. Identify $'A'$,$'B'$ and $'C'$,write the reactions involved,and draw the structures of $'B'$ and $'C'$.

Sodium hydroxide solution reacts with phosphorus to give phosphine. To bring about this reaction,we need

Which of the following statements is $NOT$ correct?

The passivity of iron in concentrated nitric acid is due to