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

Explanation

Nitrogen cannot form `dpi-ppi` multiple bond with oxygen because d - orditals do not exist in N - atom. Thus, covalency of N cannot go beyond 4. But in `R_(3)N = 0`, valency of N should be 5. So, these compounds do not exist. Whereas d - orbitals are present in phosphorus. So, P can form `dpi - ppi` multiple bonds and hence, can expand its covalency beyond 4. Therefore, P forms `R_(3)P = 0` in which the covalency of P is 5.