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-tetra­cyano­quinodi­methane) in a 1:3:2 molar ratio yielded single crystals of bis­[tris­(2,2′-bi­pyridine-κ2N,N′)nickel(II)] bis­(7,7,8,8-tetra­cyano­quinodi­methane radical anion) bi[7,7,8,8-tetra­cyano­quino­dimethanide] hexa­hydrate, [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 mol­ecules present in the crystal structure participate in hydrogen-bonding inter­actions.