A contribution to borate crystal chemistry: Rules for the occurrence of polyborate anion types

The structural features of over 460 anhydrous oxoborates that are described in literature were analysed. Lewis acid strengths were calculated for all cations other than B3+ that were found in these structures, using the valences of the cations and their determined average coordination numbers. From the observation of a restriction of the occurrence of complex polyborate anions to specific ranges of cation Lewis acid strength in existing oxoborate compounds, the Lewis base strengths of the complex polyborate anions (not of the oxygen ligands) were inferred, using the idea that is at the bottom of the valence matching principle, that in the most stable compounds the Lewis acid strength of the cation nearly equals the Lewis base strength of the anion. From this, general inherent rules for the occurrence of the miscellaneous polyborate anions with respect to the ratio cation: boron (A:B), the Lewis acid strength(s) La of the cation(s) other than B3+ and the cation radii were developed. These rules offer the possibility for a prediction of the stoichiometry and the most probable polyborate structural group that will be adopted by a new anhydrous borate compound.