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

Explanation

Nitrogen does not have vacant d-orbitals on its valence shell. Therefore, it cannot extend its covalency to five and `d pi-p pi` bonding is not possible. As a result, the molecules of `R_(3)N=O` does not exist. However, phosphorus and rest of the members of the group 15 have vacant d-orbitals in the valence shell which can be involved in `dpi- p pi` bonding. Under the circumstances, `R_(3)P=O` molecule can exist.