Why does `R_3P=O` exist but `R_3N=O` does not? (R= alkyl group)

Explanation

N due to the absence of d-orbitals can not form p`pi`-d`pi` multiple bonds. As a result N can not expand its covalency beyond four but in `R_(3)` N=O, N has a covalency of 5. So the compounds `R_(3)`N = 0 does not exist. But P due to the presence of d- orbitals form p`pi`-d`pi` multiple bonds and hence can expand its covalency beyond 4 i.e P forms `R_(3)P = O` in which covalency of P is 5.