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

Explanation

Nitrogen cannot form `ppi-dpi` multiple bonds, due to the absence of d -orbitals. As a result, the covalency of nitrogen cannot be expanded beyond 4. However, in `R_(3)N=O` , the covalency of N is 5. Hence, `R_(3)N =O` does not exist. On the other hand, phosphorus forms `ppi-dpi` multiple bonds, due to the presence of d-orbitals. As a result, P can expand its covalency beyond 4 and form `R_(3)P =O`, in which, its covalency is 5.