Question

Why does $R_{3}P$=O exist but $R_{3}N$=O does not (R=alkyl group) ?

Answer 1

Nitrogen can not form $d\pi -p\pi$ multiple bond with oxygen because d-orbitals do not exist in N-atom. Thus, the covalency of N can not go beyond 4. But in $R_{3}N$= O, the valency of N should be 5. So, these compounds do not exist.

Whereas d-orbitals are present in phosphorus. So, P can form $d\pi -p\pi$ multiple bonds and hence, can expand its covalency beyond 4. Therefore, P forms $R_{3}P$=O in which the covalency of P is 5.