The organometallic zwitterion [CoIII(η5-C5H4COOH)(η5-C5H4COO)] reacts quantitatively as a solid polycrystalline phase with a number of crystalline alkali salts MX (M = K+, Rb+, Cs+, NH4+; X = Cl−, Br−, I−, PF6−, although not in all cation/anion permutations) to afford supramolecular complexes of the formula [CoIII(η5-C5H4COOH)(η5-C5H4COO)]2⋅M+X−. In some cases, the mechanochemical complexation requires kneading of the two solids with a catalytic amount of water. The characterization of the solid-state products has been achieved by a combination of X-ray single-crystal and powder-diffraction experiments. The hydrogen-bonding interactions have been investigated by solid-state NMR spectroscopy. The mechanochemical reactions imply a profound solid-state rearrangement accompanied by breaking and forming of OH⋅⋅⋅O hydrogen-bonding interactions between the organometallic molecules. All compounds could also be obtained by solution crystallization of the inorganic salts in the presence of the organometallic unit. The solid-state complexation of alkali cations by the organometallic zwitterion has been described as a special kind of solvation process taking place in the solid state.