Proton sharing in bis(4-carbamoylpyridinium) squarate.

Reaction in aqueous solution of nickel(II) squarate with isonicotinamide yielded well formed yellow crystals of the title compound, 2C6H6N2O+.C4O4(2-), as a side product. The squarate dianion is bisected by a crystallographic twofold rotation axis, which passes through the centres of two opposite bonds of the ring. Crystal structure analysis reveals that, far from forming discrete ionic species, it is likely that there is a large degree of proton sharing between the anion and cation, with the H atom lying almost symmetrically between the donor and acceptor sites, as evidenced by the long N-H and short H...O distances [1.15 (3) and 1.39 (3) angstroms, respectively]. Other hydrogen bonding is more conventional, and there are weaker C-H...O interactions contributing additional stability to the structure.