Dilithium (citrate) crystals and their relatives

New compounds of the type LiMHC₆H₅O₇ (M = Li, Na, K, Rb) have been prepared from the metal carbonates and citric acid in solution. The crystal structures have been solved and refined using laboratory powder X-ray diffraction data, and optimized using density functional techniques. The compounds crystallize in the triclinic space group P and are nearly isostructural. The structures are lamellar, with the layers in the ab plane. The boundaries of the layers consist of hydro­phobic methyl­ene groups and very strong inter­molecular O—H...O hydrogen bonds. The O...O distances range from 2.666 Å for M = Li to 2.465 Å for M = Rb. The Li—O bonds exhibit significant covalent character, while the heavier M—O bonds are ionic. The Li atoms are four-, five-, or six-coordinate, while the coordination numbers of the larger cations are higher, i.e. eight for Na and nine for K and Rb. The citrate anion occurs in the trans,trans conformation, one of the two low-energy conformations of an isolated citrate anion. The crystal structure of LiRbHC₆H₅O₇·H₂O was also solved and refined. It consists of the same layers as in the anhydrous M = Rb compound, with inter­layer water mol­ecules and a different hydrogen-bonding pattern.