On the existence of long C–C bonds between pairs of anions which repel: when and why? A test case on the [TCNE] 2 2− dimers found in ionic crystals

Many of the Cm[TCNE]n (C = cation) salts have intradimer C–C interactions in the range of 2.9 to 3.5 Å and show the electronic fingerprints associated with C–C bond formation (IR and UV spectra, magnetic properties, structural changes), despite the fact that two [TCNE]˙− anions should repel each other due to purely Coulombic considerations. Herein, we analyze these pairwise interactions in detail for a particular crystal in which such a C–C bond is found, to understand when and why these bonds are formed in a general case.