A covalent fraction model for lanthanide compounds

We formalize the ordinal covalency scale for lanthanide compounds, introduced in a previous work, through an analytical function (the covalent fraction) of the overlap polarizability. The inverse of a parameter appearing in this function is interpreted as a typical average volume of the overlap region. The red shift in the nephelauxetic effect for the 5D0 → 7F0 transition is analyzed, for 15 europium compounds, in terms of the ligand field strength parameter, Nv, and the covalent fraction. A clear tendency of this shift to increase with the covalent fraction is observed. A similar trend, though less clear, is also observed with respect to Nv, which is interpreted as a reflection of covalent effects in the ligand field parameters.