The title compound, Rb₂[Co(H₂O)₆](C₈H₅O₄)₄·4H₂O, consists of nearly regular octahedral [Co(H₂O)₆]²⁺ cations with the CoII cations on the inversion centre (special position 2a), Rb⁺ cations, hydrogen phthalate (Hpht⁻) anions and disordered water molecules. The Rb⁺ cation is surrounded by nine O atoms from Hpht⁻ anions and water molecules, with a strongly deformed pentagonal-bipyramidal geometry and one apex split into three positions. The crystal packing is governed by numerous hydrogen bonds involving all water molecules and Hpht⁻) anions. In this way, layers parallel to the ab plane are formed, with the aromatic rings of the Hpht⁻ anions esentially directed along the c axis. While Hpht⁻ anions form the outer part of the layers, disordered water molecules and Rb⁺ cations alternate with [Co(H₂O)₆]²⁺ cations in the inner parts. The only interactions between the layers are van der Waals forces between the atoms of the aromatic rings. A search of the Cambridge Structural Database for coordination modes and types of hydrogen-bonding interaction of the Hpht⁻ anion showed that, when uncoordinated Hpht⁻ anions are present, compounds with intermolecular hydrogen bonds are more numerous than compounds with intramolecular hydrogen bonds. For coordinated Hpht⁻ anions, chelating and bridging anions are almost equally common, while monodentate anions are relatively scarce. The same coordination modes appear for Hpht⁻ anions with or without intramolecular hydrogen bonds, although intramolecular hydrogen bonds are less common.
Keywords: crystal structure; hexaaquacobalt(II); hydrogen phthalate anion; rubidium salt.