Cu(PPh3)(3)(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE](2)(2-) dimers possessing a long 2.89 +/- 0.05 A two-electron four-center (2e(-)/4c) C-C bond in the solid state. This bond arises from the overlap of the b(2g) pi* singly occupied molecular orbital (SOMO) on each [TCNE](center dot-) fragment, forming a filled bonding orbital of b(2u) symmetry, and the stabilizing effect of the cation center dot center dot center dot anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)(2)(TCNE) exhibits a related, but different, [TCNE](center dot-)center dot center dot center dot[TCNE](center dot-) motif in the solid state that lacks the 2e(-)/4c C-C bonding. To better understand the unusual nature of 2e(-)/4c C-C bonding, the genesis of the differences between their respective pi-[TCNE](center dot-)center dot center dot center dot[TCNE](center dot-) interactions was sought. The lack of 2e(-)/4c C-C bond formation is attributed to the weaker radical character of the [TCNE](center dot-) ligand, which has a total spin population of only 0.5 electron, half of that required for two S = (1)/(2) [TCNE](center dot-) moieties to form a [TCNE](2)(2-) dimer. Hence, the antiferromagnetic Mn-II-[TCNE](center dot-) intramolecular interaction (between the formally S = (1)/(2) Mn-bound [TCNE](center dot-) and the paramagnetic Mn(II)) dominates over the intermolecular pi-[TCNE](center dot-)-[TCNE](center dot-) spin coupling (between two S = (1)/(2) [TCNE](center dot-) needed to form [TCNE](2)(2-)). Therefore, by selecting specific metal ions that can interact with sigma-[TCNE](center dot-), dimerization forming [TCNE](2)(2-) can be favored or disfavored.