Comparison of Nitrogen and Phosphorus
N occurs as gaseous N2 (which is unreactive due to the high energy required to break the strong triple bond), whereas P occurs in many allotropes, including white phosphorus, P4, which all contain P-P single bonds. P4 is very reactive due to the bond strain in the tetrahedral P4 molecule.
The decreasing preference for low numbers of multiple bonds rather than high numbers of single bonds as the group is descended is a general tendency.
Ammonia, NH3, is stable, whereas phosphine, PH3, is thermodynamically unstable. This reflects the lower bond energy of P-H compared to N-H.
PH3 is not as soluble in water as NH3. This is due to the fact that NH3 forms extensive hydrogen bonding with water, and PH3 does not.
When PH3 does dissolve in water, it forms a neutral solution, whereas aqueous NH3 is basic. The enthalpy of hydration of EH4+ is greater for E=N than E=P, and again this reflects the easier formation of hydrogen bonds in the aqueous ammonia system.
In complex formation, PH3 acts as π-acceptor as well as a σ-donor. This is because P has vacant low energy orbitals, the P 3d-orbitals, available for back donation, whereas N has no such orbitals and so can act as a σ-donor only.
PX3 are made by direct combination of the elements (apart from PF3).
PCl3 is reactive, but NCl3 is endothermic and explosive. This reflects the stronger P-X bonds compared to N-X, due to extra contribution from back donation from occupied p-orbitals on the halide to form P(dπ)-X(pπ) bonds, which cannot form in N-X.
The hydrolysis products of ECl3 are different: in NCl3, the fact that B(N-H)>B(N-O), due to the presence of electrostatic repulsion between the non-bonded electrons on N and O, favours formation of ammonia. In PCl3, the fact that B(P-O)>B(P-H), due to the ability of O to form a bonding pπ-dπ interaction with P which H cannot, favours formation of phosphorus acid.
P forms PX5: there are no N equivalents.
P is more stable in the high oxidation states.
P forms many complexes with high coordination numbers, eg. six-coordinate PF6–, whereas the maximum coordination number of N is four. This tendency for hypervalence is due to the larger size of the P atom compared to N, and the presence of low-lying vacant orbitals which can participate in bonding in P but which are not present in N.
Phosphorus oxides are polymeric, made up of PO4 tetrahedra. Nitrogen oxides are molecular.
This again reflects the greater tendency to form multiple bonds at the top of the group, with B(N=O)>B(P=O), but B(P-O)>B(N-O).