A bond-valence investigation of two series of isostructural lanthanide compounds

Since the days of Pauling it has been tacitly assumed that the valence, which is distributed between the bonds to neighbouring atoms, is the stoichiometric valence, ^stoichV, which has integer values. It is shown here that this is not true. Instead, bond-valence sums (BVS) calculated for lanthanide (Ln) atoms of a series of garnet-type compounds LnTeLiO deviate significantly from ^stoichV. Values of (BVS)_Ln of this series, plotted versus the element number Z_Ln, show the same irregular sequence as: (i) the third ionization potentials of Ln atoms, (ii) the valence values previously calculated with quantum-chemical methods for lanthanide metals and sulfides, and (iii) the experimental efficiencies of the reactions between Ln^I and SF₆ in the gas phase. This indicates that in the Ln₃Te₂Li₃O₁₂ series the BVS values of the Ln atoms, which are `rattling' in rather large and rigid voids of the structure, reflect the electronic structure of Ln. It is, therefore, concluded that BVS represent a non-integer valence, which is based on the electronic structure of bonded atoms, rather than <sup>stoich</sup>V. For this non-integer valence the term `structural valence' and the symbol ^structV have been proposed. In another series of lanthanide chelates the (BVS)_Ln values deviate even more from ^stoichV_Ln = 3 v.u. than in the garnet-type series. In the chelates the Ln atoms are `squeezed' in rather small voids so that the electronic effects of Ln acting on (BVS)_Ln are overridden by stronger steric effects exerted by the organic ligands.