Hydrate isomerism in [Cu(en)2(H2O)1.935]2[Fe(CN)6].4H2O.

The title compound, bis[diaquabis(ethylenediamine2N,N0)copper(II)] hexacyanoiron(II) tetrahydrate, [Cu(C2H8N2)2(H2O)1.935]2[Fe(CN)6] 4H2O, was crystallized from an aqueous reaction mixture initially containing CuSO4, K3[Fe(CN)6] and ethylenediamine (en) in a 3:2:6 molar ratio. Its structure is ionic and is built up of two crystallographically different cations, viz. [Cu(en)2(H2O)2] 2+ and [Cu(en)2(H2O)1.87] , there being a de®ciency of aqua ligands in the latter, [Fe(CN)6] 4ÿ anions and disordered solvent water molecules. All the metal atoms lie on centres of inversion. The Cu atom is octahedrally coordinated by two chelate-bonded en molecules [mean CuÐN = 2.016 (2) AÊ ] in the equatorial plane, and by axial aqua ligands, showing very long distances due to the Jahn±Teller effect [mean CuÐO = 2.611 (2) AÊ ]. In one of the cations, signi®cant underoccupation of the O-atom site is observed, correlated with the appearance of a non-coordinated water molecule. This is interpreted as the partial contribution of a hydrate isomer. The [Fe(CN)6] 4ÿ anions form quite regular octahedra, with a mean FeÐC distance of 1.913 (2) AÊ . The dominant intermolecular interactions are cation±anion OÐH N hydrogen bonds and these interactions form layers parallel to (001).