In $R_3\mathrm{N}=\mathrm{O}$, covalency required is five. The maximum covalency ofnitrogen is four as it does not possess d-orbitals in the valence shell i.e., itcannot extend its valency beyond four. On the other hand, other membershave d-orbitals and can utilise these orbitals to show covalency of five orsix $\text{e.g., } R_3\mathrm{P}=\mathrm{O}, \mathrm{PCl}_5, [\mathrm{SbF}_6]^{-}$, etc. These can form $d\pi - p\pi$ bonds.