The nature of the chemical bond revisited. An energy partitioning analysis of diatomic molecules E₂ (E = N-Bi, F-I), CO and BF

The nature of the chemical bonds in the diatomic molecules E₂ (E = N-Bi, F-I), CO and BF has been studied with an energy partitioning analysis using gradient-corrected density functional theory calculations. The results make it possible to estimate quantitatively the strength of covalent and electrostatic attractions and the Pauli repulsion between the atoms. The data suggest that some traditional explanations regarding the strength of the molecules should be modified. The energy partitioning analysis shows that the chemical bonds in the group 15 diatomic molecules have significant electrostatic character, which increases from 30.1% in N₂ to 58.3% in Bi ₂. The contribution of the electrostatic attraction to the binding interactions in Sb₂ and Bi₂ is larger than the covalent bonding. The strength of the π bonding in the triply bonded dinitrogen is less than that of the σ bonding. The calculations indicate that ΔE_π is between 32.2% (Bi₂) and 40.0% (P ₂) of the total orbital interaction energy (ΔE_orb). The much stronger bond of N₂, as compared with the heavier group 15 E₂ homologues, is not caused by a particularly strong contribution by the π bonding, but rather by the relatively large a interactions. The comparison of N₂ with isoelectronic CO shows that the electrostatic character in the heteroatomic molecule is slightly smaller (28.8%) than in the homoatomic molecule. The contribution of the π bonding in CO is larger (49.2%) than in N₂ (34.3%). The reason why CO has a stronger bond than N₂ is the significantly weaker Pauli repulsion in CO. The electrostatic character of the bonding in BF is slightly larger (32.0%) than in CO and N₂. BF has much weaker π-bonding contributions that provide only 11.2% of the covalent interactions, which is why BF has a much weaker bond than CO and N₂. The chemical bonds in the dihalogen molecules have much higher covalent than electrostatic character. The ΔE_orb term contributes between 74.4% (Br₂) and 79.7% (F₂) to the total attractive interactions. The relatively weak bond in F₂ comes from the rather large Pauli repulsion.