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.