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

Abstract

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→⅓Ln(3+) + 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 Ln(0) and LnF3. (5) The Ln-F interaction is basically ionic (increasing with decreasing ionic radii) with some dative Ln(3+)← 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.

The electronic configurations of LnX (Ln=La–Eu, X=O, S, Se, Te): A FON–DFT investigation

“…Since our results agree with popular theoretical programs and approaches, we think this work is still acceptable and reliable. Similar DFT calculations on lanthanide systems have been reported in our recently works [26,59,60].…”

“…Such cases were also found in lanthanide trihalides [26]. An earlier G03/B3LYP investigation by Wu et al [16] in 2007 reported theoretical BE results of LnO with smaller deviation than our calculation.…”

“…So, there is a significant atomic contraction when the 4f shells are gradually filled through the lanthanide series, which is known as the lanthanide contraction. Unlike the smooth decrease of bond lengths in lanthanide trihalides [26], there is even an increase of bond lengths from CeX to EuX. This abnormal trend indicates the variation properties of Ln-X bonds, which will be discussed in Section 3.2.…”

“…Similar method was later applied to PrF 2+ and PmF 2+ and overlapping of atomic shells was observed implying possibility of 'covalent mixing' [24]. Orbital mixings of Ln(4f) have been observed in LuF 3 [25], LnF 3 (Ln = La to Lu) [26] and LnCl 6 XÀ [27], which are not evidences of bonding according to our previous discussion [26] and Pyykkö's correction [28]. Recently, Meyer [29] suggested that all the lanthanides may exist divalent with +2 oxidation in compounds.…”

“…In lanthanide trifluorides, the SOC influence on the bond lengths is very small (less than $2 pm) [26]. But in LnX series, the bond lengths are contracted from 0.1 to 5 pm by SOC effect.…”