In order to examine the possibility of Lewis acid-Lewis base (LA-LB) interactions between the boron atom of B(C6F5)(2)OH and the oxo groups ("yl" oxygen atoms) of uranyl beta-diketonato complexes, we have measured the H-1, B-11 , O-17, F-19 NMR and IR spectra of toluene solutions containing beta-diketonato complexes [UO2 (acac)(2)DMSO or UO2 (dth)(2 )DMSO, where acac = 2,4-pentanedionate, dth = 1,1,1,2,2,6,6,7,7,7-decafluoroheptane-3,5-dionate, and DMSO = dimethyl sulfoxide] and B(C6F5)(2) OH. B-11 and O-17 NMR spectra of solutions containing UO2(dth)(2) DMSO and B(C6F5)(2) OH showed no change in their chemical shifts regardless of the [B(C6F5)(2) OH]/[UO2 (dth)(2) DMSO] ratio. This indicates that there were no apparent interactions between B(C(6)F5)(2) OH and UO2(dth)(2) DMSO. On the other hand, in the corresponding NMR spectra of solutions containing UO2(acac)(2)DMSO and B(C6F5)(2) OH, new signals were observed at a higher field than signals observed in the solutions containing only B(C6F5)(2) OH or UO2(acac)(2)DMSO, and their intensity changed with the [B(C6F5)(2) OH]/[UO2(acac)(2) DMSO] ratio. These results reveal that a complex with LA-LB interaction (B center dot center dot center dot O=U) between the boron atom of B(C6F5)(2) OH and the "yl" oxygen atom of UO2(acac)(2) DMSO was formed. IR spectra also supported such complex formation; i.e., the asymmetric O=U=O stretching band of UO2(acac)(2) DMSO was observed to shift from 897 to 810 cm(-1) with the addition of B(C6F5)(2) OH. Moreover, F-19 NMR spectra indicated that 1:1 and 2:1 LA-LB complexes exist in equilibrium, UO{OB(C6F5)(2) OH}(acac)(2) DMSO + B(C6F5)(2) OH = U{OB(C6F5)(2)OH}(2) (acac)(2) DMSO. The thermodynamic parameters for this equilibrium were obtained as K = (2.5 +/- 0.6) X 10(2) M-1 (at 25 degrees C), Delta H = -42.4 +/- 5.2 kJ mor(-1), and Delta S = 96.7 +/- 19.4 J K-1 mol(-1). In H-1 NMR spectra, the signal due to -CH groups of UO2(acac)(2) DMSO disappeared, and three signals due to the corresponding -CH groups newly appeared with an increase in the [B(C6F5)(2) OH]/[UO2(acac)(2) DMSO] ratio. From these phenomena, it is proposed that 1:1 and 2:1 LA-LB complexes having interactions between the -CH groups of acac and the - OH group of coordinated B(C6F5)(2) OH are formed depending on the [B(C6F5)(2) OH]/[UO2(acac)(2) DMSO] ratio.