Dirubidium hexaaquacobalt(II) tetrakis(hydrogen phthalate) tetrahydrate and coordination modes of the hydrogen phthalate anion

The title compound, Rb2[Co(H2O)6](C8H5O4)4·4H2O, consists of nearly regular octahedral [Co(H2O)6]2+cations with the CoIIcations 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 theabplane are formed, with the aromatic rings of the Hpht−anions esentially directed along thecaxis. 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.