is found in carbon monoxide at 257 kcal/mol. The protonated forms of CO, HCN and N2 are said to have even stronger bonds, although another study argues that the use of BDE as a measure of bond strength in these cases is misleading. On the other end of the scale, there is no clear boundary between a very weak covalent bond and an intermolecular interaction. Lewis acid–base complexes between transition metal fragments and noble gases are among the weakest of bonds with substantial covalent character, with (CO)5W:Ar having a W–Ar bond dissociation energy of less than 3.0 kcal/mol. Held together entirely by the van der Waals force, helium dimer, He2, has the lowest measured bond dissociation energy of only 0.021 kcal/mol. Homolytic versus heterolytic dissociation Bonds can be broken symmetrically or asymmetrically. The former is called homolysis and is the basis of the usual BDEs. Asymmetric scission of a bond is called heterolysis. For molecular hydrogen, the alternatives are: {| style="border-spacing: 1em 0; margin-left: -1em" |- | Symmetric: | H2 → 2 H• | ΔH° = 104.2 kcal/mol (see table below) |- | Asymmetric: | H2 → H+ + H− | ΔH° = 400.4 kcal/mol (gas phase) |- | Asymmetric: | H2 → H+ + H− | ΔG° = 34.2 kcal/mol (in water) (pKaaq = 25.1) |} Note that in the gas phase, the enthalpy of heterolysis is larger than that of homolysis, due to the need to separate unlike charges. However, this value is lowered substantially in the presence of a solvent. Representative bond enthalpies The data tabulated below shows how bond strengths vary over the periodic table. There is great interest, especially in organic chemistry, concerning relative strengths of bonds within a given group of compounds, and representative bond dissociation energies for common organic compounds are shown below. See also Bond energy Electronegativity Ionization energy Electron affinity Lattice energy References Dissociation energy Binding energy