Influence of polyhedron distortions on calculated bond-valence sums for cations with one lone electron pair

In the present bond-valence model (BVM), the bond-valence parameters r₀ and b are, in general, supposed to be constant for each A–X pair and equal to 0.37 Å for all A–X pairs, respectively. For [A_i(X_j)_n] coordination polyhedra that do not deviate strongly from regularity, these suppositions are well fulfilled and calculated values for the bond-valence sums (BVS)_i are nearly equal to the whole-number values of the stoichiometric valence. However, application of the BVM to 2591 [L_i(X_j)_n] polyhedra, where L are p-block cations, i.e. cations of the 13th to 17th group of the periodic system of elements, with one lone electron pair and X = O^−II, S^−II and Se^−II, shows that r_0i values of individual [LX_n] polyhedra are correlated with the absolute value |Φ_i| of an eccentricity parameter, Φ_i, which is higher for more distorted [LX_n] polyhedra. As a consequence, calculated (BVS)_i values for these polyhedra are also correlated with |Φ_i|, rather than being numerically equal to the stoichiometric valence of L. This is interpreted as the stereochemical influence of the lone electron pair of L. It is shown that the values of the correlation parameters and the R² values of the correlation equations depend on the position of the L cation in the periodic system of elements, if the correlations are assumed to be linear. This observation suggests that (BVS)_L describes a chemical quantity that is different from the stoichiometric valence of L.