Revised values of the bond valence parameters for ^[6]Sb(V)–O and ^[3–11]Sb(III)–O

Abstract

Bond valence parameters r₀ and b have been re-determined for ^[3–11]Sb(III)–O and ^[6]Sb(V)–O, utilising crystal structures of natural and inorganic compounds from the Inorganic Crystal Structure Database. Bond valence parameters for Sb(III) were obtained from a best-fit r₀–b curve for 242 independent SbO_n polyhedra. For ^[6]Sb(V), a curve of best fitting r₀–b pairs was determined by fitting to 207 independent SbO₆ octahedra; b was then determined by optimising bond valence sums on the oxygens of Sb₂O₅ and Sb₂O₄, given the limited low quality structural data available for Sb(V) coordination numbers other than 6. Parameter values that minimised r.m.s. deviation from the ideal bond valence sums were r₀ = 1.925 Å and b = 0.455 Å for Sb(III) and r₀ = 1.904 Å, b = 0.430 Å for ^[6]Sb(V). The increase in r₀ for Sb(III) may represent the repulsive effect of the lone-pair electrons, while the difference in b indicates higher polarisability when these electrons are present. Consideration of subsets of data for differing coordination numbers demonstrates that Sb(III) parameters are applicable to all SbO_n coordination numbers (CN = 3–11). We also show that the apparent overbonding using the classical b value cannot be an artefact of unresolved site splitting. For Sb(V), independent determination of b allows bond lengths cautiously to be estimated for CN ≠ 6. This work confirms that the “universal” value b = 0.37 Å is not adequate for heavier cations such as Sb.