Plasticity of the coordination sphere of Ag2+

The geometric distortions of the ligand environment around Ag2+ cations have been examined for over 20 structurally characterized fluorides of Ag2+. Divalent silver is subject to a strong Jahn-Teller effect and it is most frequently found in an elongated or a compressed octahedral site. Typical Ag-F bond lengths vary from 2.34 to 2.60 angstrom (axial, R-ax) and from 2.01 to 2.11 angstrom (equatorial, R-eq) for the elongated octahedral site. The corresponding values for the compressed octahedral coordination are 2.00-2.09 angstrom (R-ax) and 2.20-2.42 angstrom (R-eq). The ellipsoidal Ag2+ cation exhibits significant plasticity of its coordination sphere, similar to its lighter d(9) congener, Cu2+: the dependence of R-eq vs. R-ax is inverse and monotonic. The Jahn-Teller distortion parameter D (=R-ax/R-eq) takes values from 0.83 to 1.29, with the forbidden range from 0.94 to 1.11 (close to ideal octahedron). Novel parametrization and subsequent analysis of the valence bond sums reveal that selected compounds of Ag2+ require careful structural re-examination.