On the valences of bonds in the oxycomplexes of Sn2+
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The differences between Wang and Liebau's [Wang & Liebau (2007). Acta Cryst. B63, 216-228] stoichiometric valence (atomic valence) and structural valence (bond-valence sum) observed in Sn(2+) and other lone-pair cation oxycomplexes arises from their use of the Brese & O'Keeffe bond-valence parameters which are based on the assumption that the bond-valence parameter b = 0.37 A applies to all bond types. According to the theory of the bond-valence model, the bond-valence sum is necessarily equal to the ionic charge, implying that in the Wang and Liebau model the ionic charges are equal to the structural valence. If charges are chosen equal to the stoichiometric valence, the bond-valence parameters for Sn(2+)-O bonds are R(0) = 1.859 A, b = 0.55 A. While both models are theoretically valid, only the standard model relates bond valences to the concept of atomic valence. Wang and Liebau's suggestion that cation-lone-pair bonds make a significant contribution to the valence sums is confirmed, but such bonds cannot account for the full difference between the stoichiometric and structural valences because they are present in only a few compounds.
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