A new type of lanthanide(III) (La, Gd, Yb, or Y) disalicylidene-1,a-phenylenediamine (H(2)dsp) Schiff-base complex with the formula M[Ln(dsp)(2)] (where M(+) is an alkali metal ion : Li+, Na+, K+, or Cs+) has been synthesized. Investigation of the known Ln(2)dsp(3) complexes in dimethyl sulfoxide (DMSO) solution by ultraviolet-visible absorption and proton nuclear magnetic resonance spectroscopies shows that an equilibrium Ln(2)dsp(3) reversible arrow [Ln(dsp)](+) + [Ln(dsp)(2)](-) exists in solution. The equilibrium is affected by the different size:charge ratios of the different lanthanide ions. Possible structures are suggested for M[Ln(dsp)(2)] and Ln(2)dsp(3) based on the results; e.g., Ln(2)dsp(3) apparently has one dsp(2-) acting as a bridging ligand with each of its oxygen atoms coordinated to two Ln(3+) simultaneously. The new M[Ln(dsp)(2)] complexes exhibit high thermal stability. The choices of both the alkali metal and the lanthanide ions strongly affect the thermal stability and the DMSO solubility of the M[Ln(dsp)(2)] species. The [Ln(dsp)(2)](-) ions have excellent stability in DMSO solution. Syntheses of new yttrium complexes with the formula M[Y(sal)(4)] (where M = Li+ and Na+ and sal(-) = the anion of salicylaldehyde) are also reported.