IntroductionHydrogen bonding (HB) interactions between ions have been a subject of continuing investigation in solution1and in the solid state2as well as being the subject of theoretical studies by both empirical andab initiomethods.3More recently HB interactions between ions have begun to be utilised systematically in the crystal engineering of new materials with remarkable achievements.4,5The reason for this interest stems from the fact that HB between ions combines the strength of the Coulombic field generated by the ions with the directionality and reproducible topology of hydrogen bonding interactions.6It has been pointed out that the contribution of hydrogen bonding interactions to the cohesive energy of a crystal depends on the charge carried by the hydrogen-bridged ions: if the HB joins ions of opposite charge [more generally(+)X–H⋯Y(−)] then the bonding contribution adds to the favourable (+)⋯(−) Coulombic interaction between donor and acceptor systems; whereas, if the ions carry the same charge,e.g.(−)X–H⋯Y(−)[but also(+)X–H⋯Y(+)], then the contribution reduces the repulsive electrostatic terms [i.e.(−)⋯(−) or (+)⋯(+)] arising from the Coulombic interactions between like charges. The possible combinations of ionic charges on the donor–acceptor systems are shown below: note that the only ‘true’ charge­assisted interactions are those of the(+)X–H⋯Y(−)and X–H⋯Y(−)type.When there is competition for the ‘possession’ of the H­atom,i.e.in the case of proton transfer from acid to base [e.g.X⊕+⊕H–Y⊕→⊕(+)X–H⊕+⊕Y(−)], it is the stronger acid that gives the proton away, whilst the species accepting the proton is the stronger base. This way of thinking is almost reversed when considering hydrogen bonding donation within intermolecular or interionic X–H⋯Y interactions. In this latter situation the HB donor is the electrophile (X–H), while the HB acceptor (Y) acts as the nucleophile. Considerable effort has been made to correlate concepts such as acidity and basicity to HB formation and stability.7Correlations between the pKof the acids and HB distances are available.8It has been recently argued, however, that hydrogen bond ability can be better appreciated by examining the charge distribution in the HB system rather than its acid–base chemistry.9In this paper we discuss a rather unusual competition between the polyprotic organometallic acid cation [(η5­C5H4COOH)2Co]+and the polyprotic inorganic acid H3PO4. The two acids have been co­crystallised from water. The crystalline products [(η5­C5H4COOH)2Co]+[H2PO4]−·H2O (1) and [(η5­C5H4COOH)2Co]+[H2PO4]−·[(η5­C5H4COOH)(η5­C5H4COO)Co]·2[H3PO4] (2) are essentially ionic in nature, thanks to the cationic and anionic nature of the two acids. The hydrated compound1is obtained first, while2crystallises only when the solution is concentrated by solvent evaporation. The high quality of the diffraction data allowed unambiguous definition of the H­atom positions within the O–H⋯O bridges.The supramolecular chemistry of the cobalticinium acid has been extensively studied by us.10In this earlier study our interest had been focused on the participation of the organometallic acid in hydrogen bonded networks as a mono­cation when fully protonated, as a zwitterion when mono­deprotonated and as a mono­anion when both protons are removed.