Halide-abstraction reactions of tin(IV) and lanthanide(IV) chlorides in tetrahydrofuran: Crystal and molecular structures of [LnCl₂(thf)₅][SnCl₅(thf)] where Ln = Ce, Gd or Yb

It has been established that halide abstraction from several lanthanide(III) chlorides can be effected by tin tetrachloride. Direct treatment (1:1) of LnCl₃ where Ln = Ce, Gd or Yb with SnCl₄ in tetrahydrofuran (thf) provided colourless compounds of the generic type LnSnCl₇(thf)₆ which have been identified by X-ray crystallography as ionic salts [trans-LnCl₂(thf)₅][SnCl₅(thf)]. The individual seven-co-ordinate cations feature a regular pentagonal-bipyramidal metal geometry in which a trans-LnCl₂ unit is surrounded by five thf molecules arranged in an equatorial plane. These co-ordinated solvent molecules adopt the familiar 'propeller-like' arrangement indicative of a skew as opposed to envelope ligand conformation. For Ln = Ce, Ce-Cl 2.697(2), Ce-O mean 2.495(5) Å; for Ln = Gd, Gd-Cl 2.608(2), Gd-O mean 2.415(4) Å; for Ln = Yb, Yb-Cl 2.5375(13), Yb-O mean 2.346(4) Å. The six-co-ordinate [SnCl₅(thf)]^- anions common to the series show a two-fold axis of symmetry containing the metal atom, the oxygen (thf) atom and a chloride atom mutually trans to the latter. Bond distances for Ln = Ce, i.e. Sn-O 2.276(7), Sn-Cl mean 2.399(2) Å, are typical for the series.