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

Explanation

N due to the absence of d-orbitals, cannot form `p pi - d pi` multiple bonds. As a result, N cannot expand its covalency beyond four but in `R_(3)N` = O, N has a covalency of 5. Therefore, the compound `R_(3)N` = O does not exist. In contrast, P due to the presence of d-orbitals forms `p pi - d 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.