17: Long, Multicenter Bonds in Radical Anion π-dimers

Long, multicenter C–C bonds (LMBs) are a new sub-class of bonds that can occur between a pair of neutral or charged radicals, and, similar to covalent bonds, result in a diamagnetic compound. LMBs were first established for the eclipsed, cofacial π-[TCNE]₂²⁻ (TCNE=tetracyanoethylene). A systematic analysis of the properties of the long multicenter bond in the prototypical π-[TCNE]₂²⁻ dimer, where the long, multicenter bond connects two radical anions, has been established by using high level ab initio methods. The results obtained show that LMBs share the most fundamental properties of intermolecular bonds (namely, their strength and equilibrium distance, their dependence on electrostatic and dispersion components). Additionally, they have a weak bonding component that provides characteristics of covalent bonds (namely, their origin results from the overlap of the orbitals of the interacting fragments, as well as their dependence on the spin multiplicity of the aggregate.) As a result, it is possible to talk about a n-electron m-centered intermolecular bonds, in a given state spin multiplicity. LMBs are not van der Waals bonds as they lack a bonding component. Also, they cannot be electrostatic bonds (due to the presence of an important bonding and dispersion components).