CORRELATIONS BETWEEN INDIVIDUAL BOND VALENCE PARAMETERS r₀ AND STEREOCHEMISTRY OF CATIONS WITH LONE-PAIR ELECTRONS. X. Wang and F. Liebau, Mineralogisches Institut der Universitt Kiel, 24098 Kiel, Germany.

Based on correlations found between bond valence parameters, r₀ , and the stereochemistry of lone-pair cations L, it is suggested that valences of the bonds around L consist of two terms: the classic term depending on bond lengths and a correction term taking into account the influence of the lone-pair electrons of L.

Individual _ir₀ values have been calculated for each coordination polyhedron [LX_n] using V_i =[[Sigma]]exp[( _ir₀ - D_ij)/0.37] (1), where V_i is the formal valence of L_i and D_ij are L_i-X_j bond lengths in Å [1], with L=In^I, Tl^I, Sn^II, Pb^II, P^III, As^III, Sb^III, Bi^III, S^IV, Se^IV, Te^IV, Cl^V, Br^V and I^V, and X=F^-I, Cl^-I, O^-II, S^-II and Se^-II. For each L-X pair, with a few exceptions, such _ir₀ values are found to be strongly correlated with the lengths of vectors [[Phi]]_i= -[[Sigma]]j_ij (2), where each vector j_ij is directed from L_i to X_j and has a length of exp(-D_ij /g) with g being constant. g has been set equal to 0.2Å, since this led to the best correlation coefficients R for almost all L-X pairs assuming r₀ =P|[[Phi]]|+Q (3), with P and Q being constants. Out of the 36 L-X pairs analyzed, 22 have R² values higher than 0.5, of which 9 have R² values larger than 0.8. Generally, R² is higher for L cations having higher formal valences and/or fewer electron shells, with apparent exceptions of Sn^II-F^-I, -O^-II, -S^-II for which R² values are 0.77, 0.90, 0.86, respectively, and Se^IV-O^-II for which R² is only 0.27. At present, no explanation can be given for such exceptions.

Substituting eqn.(3) into (1) leads to V_i=exp(P|[[Phi]]_i|/0.37)(exp[(Q-D)/0.37] (4). Since [[Phi]] can be considered as a measure of the retraction of lone-pair electrons from the nucleus of L, the first term on the right side of eqn.(4) represents a correction of the influence of the lone-pair electrons.

[1] Brown, I.D., Acta Crystallogr. B48 (1992) 553-572.