| Abstract
| - Cu(PPh3)3(TCNE) possesses a [TCNE]22- dimer with a long (∼2.89 Å) two-electron four-center (2e-/4c) C−C bond in the solid state, while Mn(C5H5)(CO)2(TCNE) lacks the 2e-/4c C−C bonding. From a computational study, the lack of 2e-/4c C−C bond formation is attributed to an antiferromagnetic MnII−[TCNE]•- intramolecular interaction dominating over the intermolecular π-[TCNE]•-−[TCNE]•- spin coupling (between the two S = 1/2 [TCNE]•- needed to form [TCNE]22-).
- Cu(PPh3)3(TCNE) (TCNE = tetracyanoethylene) and 14 other examples form [TCNE]22- dimers possessing a long2.89 ± 0.05 Å two-electron four-center (2e-/4c) C−C bond in the solid state. This bond arises from the overlap ofthe b2g π* singly occupied molecular orbital (SOMO) on each [TCNE]•- fragment, forming a filled bonding orbitalof b2u symmetry, and the stabilizing effect of the cation···anion interactions in the crystal that exceed the anionicrepulsion. In contrast, Mn(C5H5)(CO)2(TCNE) exhibits a related, but different, [TCNE]•-···[TCNE]•- motif in thesolid 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 π-[TCNE]•-···[TCNE]•- interactions was sought. The lackof 2e-/4c C−C bond formation is attributed to the weaker radical character of the [TCNE]•- ligand, which has atotal 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 π-[TCNE]•-−[TCNE]•- spin coupling(between two S = 1/2 [TCNE]•- needed to form [TCNE]22-). Therefore, by selecting specific metal ions that caninteract with σ-[TCNE]•-, dimerization forming [TCNE]22- can be favored or disfavored.
|
