Nitrogen has a unique ability to form $p\pi - p\pi$ multiple bonds withitself and with other elements having small size and high electronegativity.Consequently, its bond enthalpy is very high and reactivity is less.Another factor which affects the chemistry of N is the absence of$d$-orbitals in its valence shell. As a result, not only the covalency ofN is restricted to four, it can also not form $d\pi - p\pi$ bonds. P on the otherhand, does not form $p\pi - p\pi$ bonds and hence it only forms a P – P singlebond, which can easily be broken. Also, phosphorus has vacant $d$-orbitalsand can form $d\pi - d\pi$ bond with transition metals to form compoundswhich can act as ligands.