Comparative bonding analysis of N2 and P2 versus tetrahedral N4 and P4

The nature of the chemical bonds in E-2 and tetrahedral E-4 (E = N, P) has been analysed with the help of an energy decomposition method. The pi bonds in P-2 are not particularly weak. On the contrary, the contribution of P-P pi bonding to the chemical bond in P-2 is even higher than the contribution of N-N pi bonding to the chemical bond in N-2. The higher stability of P-4 (T-d) and the much lower stability of N-4 (T-d) with regard to the diatomic species come from the substantially larger weakening of the N-N sigma-bonds compared with the P-P sigma-bonds in the tetrahedral species. The sum of the six P-P sigma-bond energies in P-4 is higher than the sum of two sigma- and four pi-bonds in two P-2, but the six N-N sigma-bonds in N-4 are weaker than two sigma- and four pi-bonds in two N-2. The crucial factor that leads to the weak N-N bonds in N-4, is the rather long N-N bonds which are 32.8 % longer than in N-2. In contrast, the P-P bonds in P-4 are only 16.2 % longer than in P-2. Since the equilibrium distances in E-2 and E-4 are determined by Pauli repulsion, it can be concluded that the origin for the different stabilities of N-4 and P-4 relative to the diatomic molecules is the exchange repulsion. The nitrogen atoms encounter stronger Pauli repulsion, because the 2s and 2p valence orbitals have a similar radius while the 3s and 3p radii are more different.