Chelation of nickel(II) by citrate. The crystal structure of a nickel–citrate complex, K2[Ni(C6H5O7)(H2O)2]2·4H2O

The structure of a nickel-citrate complex, K2[Ni(C6H5O7)(H2O)2]2·4H2O, has been determined from three-dimensional X-ray data collected by counter methods. The compound crystallises in the triclinic space group P1, with Z = 1 and cell dimensions a = 6.729(1), b = 9.100(4), c = 10.594(2) Å, α = 94.86(2), β = 100.76(2), γ = 103.70(2)°. The structure was refined to a conventional R factor of 0.033 for 1804 reflections with I > 2σ(I). The complex exists as centrosymmetric dimers, with each triionised citrate ion bound as a tridentate ligand (through two carboxylate oxygens and hydroxyl oxygen) to one nickel atom, and providing a bridging bond to the second nickel atom through a third carboxylate oxygen. Octahedral coordination of the nickel atoms is completed by water molecules. Bond distances are NO (carboxyl) 2.036(3), 2.054(3) and 2.075(3)Å, NiO (hydroxyl) 2.125(3)Å, and NiOH2 2.075(3) and 2.092(3)Å. The crystal structure is characterised by a network of interactions involving potassium ions water molecules and nickel–citrate dimers.