A rare structural characterisation of the phosphomolybdate lacunary anion, [PMo11O39]7-. Crystal structures of the Ln(III) complexes, (NH4)11[Ln(PMo11O39)2]·16H2O (Ln = CeIII, SmIII, DyIII or LuIII)

The syntheses and the crystal structures of (NH4)11[LnIII(PMo11O39)2]·16H2O (LnIII= Ce (1), Sm (2), Dy (3) or Lu (4)) are reported in which an LnIII cation is sandwiched between two ʹlacunaryʹ[PMo11O39]7- anions to give a complex with eight oxygen atoms coordinated to the lanthanide centre in a twisted square antiprismatic geometry. This analogous series of complexes represents only the second time that the [PMo11O39]7- anion has been fully crystallographically characterised. All four compounds are synthesised in high yield and characterised by a range of physical and spectroscopic techniques. 31P NMR, Raman and UV/Vis/nIR spectroscopies give a clear indication that the [LnIII(PMo11O39)2]11- anion is also stable in solution. As LnIII contracts across the lanthanide series the Ln-O bond distances decrease and the splitting of the (nu)P-O vibrational mode within the [PMo11O39]7- unit increases. The relative stability of this species in solution may have importance in elucidating the speciation of Ln(III) and An(III) cations in nuclear waste solutions where phosphopolymolybdate anions are known to form.