Coordination behaviour and two-dimensional-network formation in poly[[mu-aqua-diaqua(mu5-propane-1,3-diyldinitrilotetraacetato)dilithium(I)cobalt(II)] dihydrate]: the first example of an M(II)-1,3-pdta complex with a monovalent metal counter-ion.

The title compound, {[CoLi2(C11H14N2O8)(H2O)3].2H2O}n, constitutes the first example of a salt of the [M(II)(1,3-pdta)](2-) complex (1,3-pdta is propane-1,3-diyldinitrilotetraacetate) with a monopositive cation as counter-ion. Insertion of the Li+ cation could only be achieved through application of the ion-exchange column technique which, however, appeared unsuccessful with other alkali metals and the ammonium cation. The structure contains two tetrahedrally coordinated Li+ cations, an octahedral [Co(1,3-pdta)](2-) anion and five water molecules, two of which are uncoordinated, and is built of two-dimensional layers extending parallel to the (010) lattice plane, the constituents of which are connected by the coordinate bonds. O-H(water)...O hydrogen bonds operate both within and between these layers. The crystal investigated belongs to the enantiomeric space group P2(1) with only one (Lambda) of two possible optical isomers of the [Co(1,3-pdta)](2-) complex. A possible cause of enantiomer separation during crystallization might be the rigidification and polarization of the [M(1,3-pdta)](2-) core, resulting from direct coordination of Li+ cations to three out of four carboxylate groups constituting the 1,3-pdta ligand. The structure of (I) differs considerably from those of the other [M(II)(1,3-pdta)](2-) complexes, in which the charge compensation is realized by means of divalent hexaaqua complex cations. This finding demonstrates a significant structure-determining role of the counter-ions.