Bond-valence sums for Tc–O systems from EXAFS data

Literature data for structures containing exclusively Tc–O bonds were used to calculate unit-valence parameters R0 for Tc(VII), Tc(VI), Tc(V) (five-coordinate), Tc(V) (six-coordinate), Tc(IV) and Tc(III) as 1.909, 1.955, 1.870, 1.859, 1.841 and 1.768 Å, respectively. A second method of estimating R0 was developed to validate the calculated values for these oxidation states because crystallographic data are limited. The method was first tested and shown to be valid using literature data for Cr, Mn, Fe and Co complexes. The validated R0 values for Tc were used to calculate bond-valence sums (BVS) for Tc solids and aqueous solutions using EXAFS data for the bond distances and coordination numbers. The calculated BVS showed good agreement with the expected values for the assumed Tc oxidation states.