Statement - I :N₃H exists but not P₃H
Statement - II :Nitrogen can form effective (P-d) pi-bonds where as phosphorus cannot form such bonds

Hydrazoic acid is N₃H. It is also known as hydrogen azide or azoimide. This acid contains azide ion which has $\mathrm{p\pi }-\mathrm{p\pi } \mathrm{bonds}.$ P₃H does not exists because P cannot form $\mathrm{p\pi }-\mathrm{p\pi } \mathrm{bonds}.$ 

Also Nitrogen has no d-orbitals, hence cannot form (p-d) pi bonds.

This means Statement-I is correct and statement-II is incorrect.