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

The nature of the chemical bonds in E₂ and tetrahedral E₄ (E = N, P) has been analysed with the help of an energy decomposition method. The π bonds in P₂ are not particularly weak. On the contrary, the contribution of P-P π bonding to the chemical bond in P₂ is even higher than the contribution of N-N π bonding to the chemical bond in N₂. The higher stability of P₄ (T_d) and the much lower stability of N₄ (T_d) with regard to the diatomic species come from the substantially larger weakening of the N-N σ-bonds compared with the P-P σ-bonds in the tetrahedral species. The sum of the six P-P σ-bond energies in P₄ is higher than the sum of two σ- and four pbonds in two P₂, but the six N-N σ-bonds in N₄ are weaker than two σ- and four π-bonds in two N₂. The crucial factor that leads to the weak N-N bonds in N₄, is the rather long N-N bonds which are 32.8 % longer than in N₂. In contrast, the P-P bonds in P4 are only 16.2 % longer than in P₂. Since the equilibrium distances in E₂ and E₄ are determined by Pauli repulsion, it can be concluded that the origin for the different stabilities of N4 and P₄ 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.