Why does `R_3 P=O` exist but `R_3 N =O` does not (R-alkyl group) ?

Explanation

The compound `R_3 P` -O can be represented as follows.
`R-underset(R)underset(|)overset(R)overset(|)P-O`
In this compound, phosphorus shows a covalency of five including a `dpi -p pi` bond with oxygen.
A similar compound of nitrogen i.e., `R_3 P = Q` is not possible because
(a) nitrogen can show a maximum covalency of four.
(b) due to the absence of d-orbitals in the valency shell, nitrogen cannot take part in `dp-p pi` bonding.