Disorder of the dimeric TCNQ–TCNQ unit in the crystal structure of [Ni(bpy)3]2(TCNQ–TCNQ)(TCNQ)2·6H2O (TCNQ is 7,7,8,8-tetra­cyano­quinodi­methane)

Crystallization from an aqueous methanol system composed of Ni(NO3)2, 2,2'-bipyridine (bpy) and LiTCNQ (TCNQ is 7,7,8,8-tetra-cyano-quinodi-methane) in a 1:3:2 molar ratio yielded single crystals of bis-[tris-(2,2'-bi-pyridine-κ2N,N')nickel(II)] bis-(7,7,8,8-tetra-cyano-quinodi-methane radical anion) bi[7,7,8,8-tetra-cyano-quino-dimethanide] hexa-hydrate, [Ni(C10H8N2)3]2(C24H8N8)(C12H4N4)2·6H2O or [Ni(bpy)3]2(TCNQ-TCNQ)(TCNQ)2·6H2O. The crystal structure comprises [Ni(bpy)3]2+ complex cations, two centrosymmetric crystallographically independent TCNQ ·- anion radicals with π-stacked exo groups, and an additional dimeric TCNQ-TCNQ unit which comprises 75.3 (9)% of a σ-dimerized (TCNQ-TCNQ)2- dianion and 24.7 (9)% of two TCNQ·- anion radicals with tightly π-stacked exo groups. The title complex represents the first example of an NiII complex containing a σ-dimerized (TCNQ-TCNQ)2- dianion. Disordered solvent water mol-ecules present in the crystal structure participate in hydrogen-bonding inter-actions.