Electronic structures of highly symmetrical compounds of f elements: XXXIX. Do more covalent europium(III) compounds exhibit an anti-nephelauxetic behavior?

By comparing the nephelauxetic ratios β of a number of molecular PrIII compounds, and the Slater parameters F2 (or Racah parameters E1) of molecular NdIII and selected SmIII compounds, presumably more covalent types of EuIII compounds could be identified. Powdered [Eu{N(SiMe3)2}3] (1), “[Eu{OC(tBu)3}3]” (“3”) and [Eu(η5-C5H5)3(CNC6H11)] (5) were resynthesized following usual procedures. The absorption transitions 7F0 → 5D0 of an oriented single crystal of 1, a glassy frozen solution of 1 dissolved in a mixture of 2-MeTHF/THF (ratio 3:1) ([Eu{N(SiMe3)2}3(THF)], (2)), of “3” dissolved in 2-MeTHF ([Eu{OC(tBu)3}3(MeTHF)], (4)), and of 5 dissolved in a mixture of the inert solvents methylcyclohexane/toluene (ratio 1:1), were measured at room and low temperatures (90 K). The energy differences of this transition for compounds 1, 2, 4 and 5 are larger than those of [Eu(H2O)9]3+ or even gaseous Eu3+, indicating quasi “anti-nephelauxetic” effects. Crystal field calculations, however, reveal that lower Slater parameters F2 (or Racah parameters E1) have to be used than those of [Eu(H2O)9]3+ in order to reproduce the experimental energy differences between 7F0 and 5D0, thus indicating the expected nephelauxetic effects of more covalent EuIII compounds.