Structures of dipotassium rubidium citrate monohydrate, K(2)RbC(6)H(5)O(7)(H(2)O), and potassium dirubidium citrate monohydrate, KRb(2)C(6)H(5)O(7)(H(2)O), from laboratory X-ray powder diffraction data and DFT calculations

The crystal structures of the isostructural compounds dipotassium rubidium citrate monohydrate, K(2)RbC(6)H(5)O(7)(H(2)O), and potassium dirubidium citrate monohydrate, KRb(2)C(6)H(5)O(7)(H(2)O), have been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. The compounds are isostructural to K(3)C(6)H(5)O(7)(H(2)O) and Rb(3)C(6)H(5)O(7)(H(2)O), but exhibit different degrees of ordering of the K and Rb cations over the three metal-ion sites. The K and Rb site occupancies correlate well to both the bond-valence sums and the DFT energies of ordered cation systems. The MO(6) and MO(7) coordination polyhedra share edges to form a three-dimensional framework. The water mol­ecule acts as a donor in two strong charge-assisted O—H⋯O hydrogen bonds to carboxyl­ate groups. The hydroxyl group of the citrate anion forms an intra­molecular hydrogen bond to one of the central carboxyl­ate oxygen atoms.