Synthesis and Characterization of LnF(HF)(BF₄)₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy), and Crystal Structures of LnF(HF)(BF₄)₂ (Ln = Pr, Nd) and La(BF₄)₃

<jats:title>Abstract</jats:title><jats:p>Rare earth trifluorides (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy) react with boron trifluoride in anhydrous hydrogen fluoride (aHF) at room temperature. Products were found to be only sparingly soluble in aHF where the solubility decreases from lanthanum to dysprosium. After an excess of BF<jats:sub>3</jats:sub> and aHF were pumped away at ambient temperature, LnF(HF)(BF<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub> type of compounds were isolated. Single crystals of LnF(HF)(BF<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub> (Ln = Pr, Nd) and La(BF<jats:sub>4</jats:sub>)<jats:sub>3</jats:sub> were grown for 3–5 months from solutions of LnF<jats:sub>3</jats:sub> (Ln = La, Pr) and Nd<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub> in aHF under pressure of BF<jats:sub>3</jats:sub> (4 bar). Structures consist from the zig‐zag [–Ln–F–Ln–] (Ln = Pr, Nd) chains connected via BF<jats:sub>4</jats:sub> units and HF molecules into tridimensional network. The Ln–F(Ln) distances are significantly shorter [2.2489(6) Å for Pr and 2.234(1) Å for Nd] than Ln–F(BF<jats:sub>4</jats:sub><jats:sup>–</jats:sup>) and Ln–F(HF) ones [Pr: 2.461(4)–2.528(5) Å, Nd : 2.446(7)–2.511(9) Å]. In La(BF<jats:sub>4</jats:sub>)<jats:sub>3</jats:sub>, the lanthanum atoms are coordinated by nine fluorine atoms with seven shorter La–F distances [2.410(2)–2.485(2) Å] and two elongated ones [2.600(2) Å]. The vibrational spectra of isolated rare earth tetrafluoroborates show strong deviations of the [BF<jats:sub>4</jats:sub>]<jats:sup>–</jats:sup> anions from ideal <jats:italic>T<jats:sub>d</jats:sub></jats:italic> symmetry. Because of the strong cation‐anion interactions, the splittings of the anion vibrational modes are observed indicating rather covalent character of the bonding between LnF<jats:sub>3</jats:sub> and BF<jats:sub>3</jats:sub>.</jats:p>