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

Explanation

Phosphorus has d-orbitals in its valance shell. Hence, it can expand its covalence and forms `p pi - d pi` bonds. Thus, `R_3P = O` is known to exist.
On the contrary, the nitrogen has maximum covalence of 4 because of absence of d-orbitals Hence, it cannot form five covalent bonds. Thus `R_3N = O` is not known.