A series of nine complexes and 109 literature examples containing a carboxylic acid functional group and a pyridine functional group on separate molecules follow the ΔpK a rule such that proton transfer occurs at values above 3 to form a molecular salt and none at values below 0 to form a co-crystal. In the intermediate range, there is a predominance of molecular salt over co-crystal formation. The complexes discussed show that calculated pK a 's are good predictors of the outcome.
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