On structure and bonding of lanthanoid trifluorides LnF3 (Ln = La to Lu)†

The trends in the series of lanthanoid (lanthanide) trifluoride molecules LnF3 (Ln = La to Lu) are governed by the valence-active Ln(4f,5d,5p,6s) shells. The series is investigated by quasi-relativistic density functional theory at both the scalar and spin–orbit-coupled levels. Integrating many of the previous experimental and theoretical deductions, we obtain the following comprehensive picture: (1) The comparatively small Ln–F bond length contraction of 14 pm from La to Lu is rather smooth but weakly modulated by spin–orbit coupling. (2) From La to Lu the floppy structure becomes more quasi-planar. (3) The heterolytic LnF bond energies (⅓LnF3 → ⅓Ln3+ + F−) at the spin–orbit averaged level increase smoothly from 15.3 to 16.3 eV for La to Lu, only the ‘divalent’ lanthanoids Eu and Yb are outliers with 0.2 eV higher bond energies. (4) The homolytic LnF bond energies (⅓LnF3 → ⅓Ln + F) however show an overall W-shaped double-periodicity with maxima for LaF3, GdF3 and LuF3, decreasing from La to Eu and from Gd to Yb, the large individual variations being caused by different spin–orbit coupling and Coulomb interaction effects in Ln0 and LnF3. (5) The Ln–F interaction is basically ionic (increasing with decreasing ionic radii) with some dative Ln3+ ← F− bonding. (6) The latter is of the Ln(5d)–F(2p) type with a rather constant bond order from La to Lu, with small Ln(5p) and very small Ln(4f) semi-core contributions decreasing from La to Lu. All these trends are rationalized.