Complex formation of the uranyl ion (UO(2)(2+)) with triphenylphosphine oxide (OPPh(3)) in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ([BMI][NfO]) ionic liquid was investigated by means of (31)P NMR spectroscopy. In [BMI][NfO], coordination of OPPh(3) to UO(2)(2+) was found, and its coordination number was 4.1 +/- 0.2, indicating UO(2)(OPPh(3))(4)(2+). From the [BMI][NfO] solution containing UO(2)(2+) and OPPh(3), yellow crystals of UO(2)(OPPh(3))(4)(ClO(4))(2) deposited, and its molecular and crystal structures were determined by using single-crystal X-ray analysis. An OPPh(3) exchange reaction of UO(2)(OPPh(3))(4)(2+) in [BMI][NfO] was also examined. The apparent first-order rate constant (k(obs)) showed the first-order dependence on [OPPh(3)] (k(obs) = k(4)[OPPh(3)](free)), suggesting the "associative" (A) mechanism. Its activation parameters were DeltaH(4)(++) = 55.3 +/- 2.8 kJ mol(-1) and DeltaS(4)(++) = 16.1 +/- 7.9 J mol(-1) K(-1). To compare the reactivity of UO(2)(OPPh(3))(4)(2+) in [BMI][NfO] with that in an ordinary organic solvent, the same reaction in CD(2)Cl(2) was studied. In the CD(2)Cl(2) system, an equilibrium between UO(2)(OPPh(3))(4)(2+) and UO(2)(OPPh(3))(5)(2+) was observed. The k(obs) values of the OPPh(3) exchange reactions in UO(2)(OPPh(3))(4)(2+) (k(4obs)) and UO(2)(OPPh(3))(5)(2+) (k(5obs)) in CD(2)Cl(2) are expressed as k(4obs) = k(4)[OPPh(3)](free) and k(5obs) = k(5), respectively, indicating that the exchange reactions in UO(2)(OPPh(3))(4)(2+) and UO(2)(OPPh(3))(5)(2+) are categorized in A and "dissociative" (D) mechanisms, respectively. The activation parameters of these reactions were also estimated (UO(2)(OPPh(3))(4)(2+): DeltaH(4)(++) = 7.1 +/- 0.3 kJ mol(-1) and DeltaS(4)(++) = -122 +/- 1 J mol(-1) K(-1), UO(2)(OPPh(3))(5)(2+): DeltaH(5)(++) = 62.4 +/- 1.0 kJ mol(-1) and DeltaS(5)(++) = 68.4 +/- 4.2 J mol(-1) K(-1)). A large difference in the reactivity of UO(2)(OPPh(3))(4)(2+) was found between [BMI][NfO] and CD(2)Cl(2). This was explained by the formation of a specific solvation barrier of NfO(-) around UO(2)(OPPh(3))(4)(2+) in [BMI][NfO].