fragment is slightly higher in the dianion (-75.7 kcal/mol) than in the dication (-69.2 kcal/mol) and the covalent (orbital) contribution ∆Eorb to the bond is a bit smaller than the electrostatic (Coulomb) attraction ∆Eelstat. The orbital interactions come in both species mainly from backdonation ∆Eorb(1) and -donation ∆Eorb(2) between Hg and [Fe(CO)n]2 2+ , which involve the occupied 6s and vacant 6pσ AOs of Hg. But the division of the σ interaction into two terms ∆Eorb(1) and ∆Eorb(2) comprises a physically connected orbital interaction where the 6s and 6pσ