The reactions of lanthanide nitrates, Ln(NO3)(3), with bis(diphenylphosphino)methane dioxide, Ph2P(O)CH2P(O)Ph-2 (L), lead to complexes with three distinct classes of structure. At low ratios of Ln:L (<1:1.5) in acetonitrile the ionic complexes [Ln(NO3)(2)L-2](+)[Ln(NO3)(4)L](-) (Ln = Pr, Eu) have been isolated. When carried out with a 1:2 or higher ratio in ethanol the reaction yields Ln(NO3)(3)L-2 (Ln = La,Ce) and [Ln(NO3)(2)L2H2O](+)[NO3](-) (Ln = Nd, Gd, Ho). Geometrical isomerism is found for the cations [Ln(NO3)(2)L2H2O](+) and is attributed to the extent of hydrogen bonding to the coordinated water. Ligand redistribution occurs on heating in the solid state giving yellow solids in all cases. Crystallization of these materials from ethanol or acetonitrile gives [Ln(NO3)L-3](2+).2[NO3](-), which have been structurally characterized for Ln = Gd and Yb. Electrospray mass spectra indicate that extensive ligand exchange reactions occur in solution.