The tetracyanopyrazinide dimer dianion, [TCNP]2(2-). 2-Electron 8-center bonding

The reaction of Cr(C(6)H(6))(2) and 1,2,4,5-tetracyanopyrazine (TCNP) forms [Cr(C(6)H(6))(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) A. The intradimer C...C and N...N separations are 3.29(2) and 3.42(2) A, 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](*-)...[TCNP](*-) 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](*-)...cation(+) electrostatic interactions, whose sum (-209.8 kcal/mol) exceeds the sum of the repulsive [TCNP](*-)...[TCNP](*-) and cation(+)...cation(+) interactions (140.3 kcal/mol).