Charge-Transfer Self-Complex Formed in Potassium Squarate Monohydrate

The x-ray crystal structure analysis of the monohydrate of the potassium salt of squaric acid is reported. Squaric acid is the name given to 3,4-diketocyclobutenediol. The main feature of the structure lies in the packing of the squarate anions. These are arranged in stacks with the planes of the anions almost parallel. Two of the carbon atoms, on neighboring anions, approach one another very closely, their separation being 3.237 Å (σ=0.012 Å). It is concluded that the structure represents a charge-transfer self-complex. The anion was found to have the expected D4h symmetry with mean CO bond length of 1.259 Å (σ=0.007 Å) and mean CC bond length of 1.457 Å (σ=0.008 Å).