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-1 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.