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 SbOn 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 Sb 2O5 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 SbOn 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.