The treatment of HfCl4 with NaLOEt (L-OEt(-) = [(eta(5)-C5H5)Co{P(O)(OEt)(2)}(3)](-)) in nitric acid afforded LOEtHf(NO3)(3) (1). Hydrolysis of 1 in acetone/water (4:1, v/v) yielded the hydroxy-bridged dimer [(L-OEt)(2)Hf-2(H2O)(4)(mu-OH)(2)][NO3](4) (2). The treatment of (NH4)(2)[Ce(NO3)(6)] with 2 equiv of NaLOEt in water afforded (L-OEt)(2)Ce(NO3)(2) (3), whereas that with 1 equiv of NaLOEt in CH2Cl2 gave LOEtCe(NO3)(3) (4). While 4 is stable in organic solvents such as acetone, it was converted completely to 3 in acetone/water. The treatment of 3 with bis(p-nitrophenyl)phosphate (BNPP) afforded (L-OEt)(2)Ce[eta(1)-OPO(OR)(2)](2) (5; R = p-NO2C6H4), whereas the reaction of 4 with NaPO2(OR)(2) yielded dinuclear [LOEtCe(NO3)(2){mu-O2P(OR)(2)}](2) [R = p-NO2C6H4 (6), Ph (7)]. The addition of 3 in acetone to an aqueous solution of NaH2PO4 yielded the cerium(IV) dihydrogen phosphate complex (L-OEt)(2)Ce(PO4H2)(2) (8). Complexes 1-5 and 7 have been characterized by X-ray crystallography. The hydrolysis of BNPP with LOEtM(NO3)(3) (M = Zr, Hf, Ce) in acetone/water has been studied by H-1 NMR spectroscopy. At 25 degrees C, with [Ce] = 20[BNPP], in an acetone-d(6)/N-(2-hydroxyethyl)piperazine-N'-2-ethanesulonic acid (4:1, v/v) buffer solution (50 mM) (4:1, v/v), the hydrolysis of BNPP with 4 was found to exhibit first-order kinetics with a rate constant of (1.1 +/- 0.1) x 10(-3) s(-1).