| Summary: | Bis(diphenylphosphino)methane dioxide (dppmO<sub>2</sub>) forms eight-coordinate cations [M(dppmO<sub>2</sub>)<sub>4</sub>]Cl<sub>3</sub> (M = La, Ce, Pr, Nd, Sm, Eu, Gd) on reaction in a 4:1 molar ratio with the appropriate LnCl<sub>3</sub> in ethanol. Similar reaction in a 3:1 ratio produced seven-coordinate [M(dppmO<sub>2</sub>)<sub>3</sub>Cl]Cl<sub>2</sub> (M = Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), whilst LuCl<sub>3</sub> alone produced six-coordinate [Lu(dppmO<sub>2</sub>)<sub>2</sub>Cl<sub>2</sub>]Cl. The complexes have been characterised by IR, <sup>1</sup>H and <sup>31</sup>P{<sup>1</sup>H}-NMR spectroscopy. X-ray structures show that [M(dppmO<sub>2</sub>)<sub>4</sub>]Cl<sub>3</sub> (M = Ce, Sm, Gd) contain square antiprismatic cations, whilst [M(dppmO<sub>2</sub>)<sub>3</sub>Cl]Cl<sub>2</sub> (M = Yb, Dy, Lu) have distorted pentagonal bipyramidal structures with apical Cl. The [Lu(dppmO<sub>2</sub>)<sub>2</sub>Cl<sub>2</sub>]Cl has a <i>cis</i> octahedral cation. The structure of [Yb(dppmO<sub>2</sub>)<sub>3</sub>(H<sub>2</sub>O)]Cl<sub>3</sub><b>·</b>dppmO<sub>2</sub> is also reported. The change in coordination numbers and geometry along the series is driven by the decreasing lanthanide cation radii, but the chloride counter anions also play a role.
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