BRIEF COMMUNICATION-Reinvestigation of CuNbOF5.4H2 O

Bond valence sums can be very useful tools for the analysis of crystallographic models (1, 2). Many examples may be cited where incorrect crystal structures were corrected by the use of this simple yet powerful technique (3, 4). However, if there is unrecognized disorder in a crystal structure, its effects on the observed bond lengths can make the application of bond valence sums difficult and lead to incorrect conclusions about the structure. A recent example of this problem is found in the compound CuNbOF 5 · 4H 2 O. Its structure was originally reported by Fischer et al. (5, 6) to consist of chains of alternating corner sharing [NbF 4 (F/O) 2@2 ]0.5~ and [Cu(H 2 O) 4 (F/O) 2@2 ]0.5` octahedra (the X /@2 notation indicates a bridging ligand), isostructural with CuTiF 6 · 4H 2 O. Recently, the compound was reinvestigated by Fourquet and co-workers (7). Their careful crystallographic analysis confirmed the CuTiF 6 · 4H 2 O structure type. However, since oxygen and fluorine are essentially equivalent in X-ray diffraction and protons are difficult to locate, the assignment of O2~, OH~, H 2 O, or F~ on any of the ligand sites in the structure is equally valid in terms of the refinement. To overcome this problem, Fourquet attempted to use bond valence calculations to identify which of the possible ligands occupied each position. Their calculations appeared to rule out the presence of O2~ and it was proposed that the octahedra were actually [Nb(OH/F) 4 F 2@2 ]0 and [Cu(H 2 O) 3 (OH/F) 1 F 2@2 ]0 corresponding to an overall formula of CuNb(OH) F · 3H O (x+3). x 7~x 2