[Cu2(HF2)(H2O)8][FeF6]·2H2O

The title compound, octaaqua(hydrogenfluorido)dicopper(II) hexafluoridoferrate(III) dihydrate, was synthesized under hydrothermal conditions. The Cu atom is coordinated by one F and five O atoms within a highly distorted octahedron, forming dimeric [Cu2(H2O)8HF2]3+ units by edge sharing. These units are hydrogen bonded to [FeF6]3− anions and to an interstitial water molecule. The former feature Fe3+ on a special position (). The dimeric copper units are linked to adjacent dimers by F—H⋯F hydrogen bonds. Additional O—H⋯O and O—H⋯F hydrogen bonds help to consolidate the crystal packing.

The title compound, octaaqua(hydrogenfluorido)dicopper(II) hexafluoridoferrate(III) dihydrate, was synthesized under hydrothermal conditions. The Cu atom is coordinated by one F and five O atoms within a highly distorted octahedron, forming dimeric [Cu 2 (H 2 O) 8 HF 2 ] 3+ units by edge sharing. These units are hydrogen bonded to [FeF 6 ] 3À anions and to an interstitial water molecule. The former feature Fe 3+ on a special position (1). The dimeric copper units are linked to adjacent dimers by F-HÁ Á ÁF hydrogen bonds. Additional O-HÁ Á ÁO and O-HÁ Á ÁF hydrogen bonds help to consolidate the crystal packing.
Such asymmetrical F-H···F hydrogen bonding was observed in many cases for F-F distances going up to 2.686 Å in [(η 5 -C 5 Me 5 )NbF 4 (HF)AsF 3 ] 2 (Roesky et al., 1990) The charge of the cations is balanced by the centrosymmetric anion [FeF 6 ] 3-. There are 14 water molecules around the (FeF 6 ) 3octahedron, the H(51) atom corresponds to a bifurcated hydrogen bond towards F(2) and F(3) (figure 3). This helps the bond valence calculations to provide relatively satisfying results (Table 3), the largest disagreements being on O(4) and F(4). The contribution from the long Cu-Ow(3) distance is negligible.
Comparing with the Cu 3 Fe 2 F 12 (H 2 O) 12 crystal structure (Kummer & Babel, 1987), also triclinic, there are strong differences. The three copper atoms and two Fe atoms are all placed on inversion centers. One (FeF 6 ) 3octahedron is isolated, and the other forms chiolite-like square meshes by sharing F corners with tetra-hydrated Cu(H 2 O) 4 F 2 elongated octahedra (the long distances are the two Cu-F ones, close to 2.32 Å for the three independent copper sites). In CuFe 2 F 8 (H 2 O) 2 (Leblanc & Ferey, 1990), the CuF 4 (H 2 O) 2 octahedra show two long Cu-F distances (2.451 Å). So, in both cases, the long distances are Cu-F ones, which is not the case of the title compound.
A study of the magnetic properties is currently in progress.  (Kummer & Babel, 1987) (5M solution) or to the title compound (1M).
Both compounds are occurring with the same intense blue color and could have been confused if no powder pattern had been recorded. At other starting compositions, mixtures of both compounds could be observed, and also together especially with Pb 8 CuFe 2 F 24 (starting from 2PbF 2 /CuF 2 /2FeF 3 for instance), isostructural with Pb 8 MnFe 2 F 24 (Le Bail & Mercier, 1992).