Synthetic, structural, and reactivity results are presented on neodymium and lanthanum carboxylates that can function as models for diene polymerization catalysts. The neodymium and lanthanum carboxylates {Nd[O2CC(CH3)2CH2CH3]3}x (1) and {La[O2CC(CH3)2CH2CH3]3}x(2) were synthesized from the lanthanide metal and 2,2-dimethylbutyric acid in the presence of a catalytic amount of HgCl2 and Hg(OAc)2. Nd2[O2CCH(C6H5)CH2CH3]6[HO2CCH(C6H5)CH2CH3]4(3) was prepared similarly from neodymium powder and 2-phenylbutyric acid. 1 and 2 crystallize from hot pyridine as the dimeric pyridine adducts Nd2[O2CC(CH3)2CH2CH3]6[pyridine]4 (4) and La2[O2CC(CH3)2CH2CH3]6[pyridine]4 (5). 3 crystallizes from hexanes as the carboxylic acid adduct. 1 and 2 react with Et2AlCl to form the isolable solids 6 and 7, which react with Al(iBu)3 to generate catalytically active systems which polymerize isoprene, ethylene, and styrene. Although crystallization of 6 from THF forms NdCl3(THF)4 (8) and crystallization of 7 from pyridine forms [LaCl3(pyridine)4]2 (9), complexes 6 and 7 are not simply LnCl3, since elemental analyses show that they contain aluminum.