Synthesis, properties and solution speciation of lanthanide chloride complexes of triphenylphosphine oxide

The reaction of Ph₃PO with LnCl₃ · nH₂O (Ln=La-Lu ≠ Pm) in a 3.5:1 ratio in acetone produces [LnCl₃(Ph₃PO)₃], whilst from a 6:1 ratio in ethanol the products are [LnCl₂(Ph₃PO)₄]Cl · n(solvate). In the presence of [NH₄][PF₆] in ethanol solution, [LnCl₂(Ph₃PO)₄]PF₆ can be isolated. The last complexes are stable in solution but the [LnCl₃(Ph₃PO)₃] and [LnCl₂(Ph₃PO)₄]Cl partially interconvert in non-coordinating solvents, the neutral species being preferred by the lighter lanthanides, the cationic tetrakis complexes becoming more favoured towards the end of the series. The complexes have been characterised in the solid state by analysis and IR spectroscopy and in solution by ³¹P{¹H} NMR spectroscopy and conductance measurements. The crystal structures of trans-[LnCl₂(Ph₃PO)₄]Cl · nEtOH (Ln=Tb or Yb) and mer-[LnCl₃(Ph₃PO)₃] · 0.5Me₂CO (Ln=La or Ce) are reported and discussed.