In R3N=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 e.g.,R3P=O,PCl5,[SbF6]–, etc. These can form dπ – pπ bonds.