The Tetracyanopyrazinide Dimer Dianion, [TCNP]22-• 2-Electron 8-Center Bonding

The reaction of Cr(C6H6)(2) and 1,2,4,5-tetracyanopyrazine (TCNP) forms [Cr(C6H6)(2)][TCNP], and TCNP is reduced and forms the eclipsed pi-[TCNP](2)(2-) dimer. Diamagnetic [TCNP](2)(2-) has an intradimer separation of 3.14(2) angstrom. The intradimer C center dot center dot center dot C and N center dot center dot center dot N separations are 3.29(2) and 3.42(2) angstrom, respectively, and increase with the distance from the center of the molecule, due to nitriles bending away from the plane of the molecule by 5 +/- 1 degrees. [TCNP](2)(2-) is best described by an atoms-in-molecules analysis as. having a 2e(-)/8c C-C bond involving the four C atoms from each six-member ring. The results of B3LYP/6-31+G(d)computed interactions indicate that the [TCNP](center dot-)center dot center dot center dot[TCNP](center dot-) interactions in an isolated [TCNP](2)(2-) are repulsive by 58.9 kcal/mol, and that the stability of [TCNP](2)(2-) primarily originates from [TCNP](center dot-)center dot center dot center dot cation(+) electrostatic interactions, whose sum (-209.8 kcal/mol) exceeds the sum of the repulsive [TCNP](center dot-)center dot center dot center dot[TCNP](center dot-) and cation(+)center dot center dot center dot cation(+) interactions (140.3 kcal/mol).