Control of two-electron four-center (2e-/4c) C-C bond formation observed for tetracyanoethenide dimerization, [TCNE]2(2-).

Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- 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 b2g pi* singly occupied molecular orbital (SOMO) on each [TCNE]- fragment, forming a filled bonding orbital of b2u symmetry, and the stabilizing effect of the cation...anion interactions in the crystal that exceed the anionic repulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]-...TCNE]- 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]-...TCNE]- interactions was sought. The lack of 2e-/4c C-C bond formation is attributed to the weaker radical character of the [TCNE]- ligand, which has a total spin population of only 0.5 electron, half of that required for two S = 1/2 [TCNE]- moieties to form a [TCNE]22- dimer. Hence, the antiferromagnetic MnII-[TCNE]- intramolecular interaction (between the formally S = 1/2 Mn-bound [TCNE]- and the paramagnetic Mn(II)) dominates over the intermolecular pi-[TCNE]--[TCNE]- spin coupling (between two S = 1/2 [TCNE]- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that can interact with sigma-[TCNE]*-, dimerization forming [TCNE]22- can be favored or disfavored.