Complex formation of the uranyl ion (UO22+) with triphenylphosphine oxide (OPPh3) in 1-butyl-3-methylimidazolium nonafluorobutanesulfonate ([BMI][NfO]) ionic liquid was investigated by means of P-31 NMR spectroscopy. In [BMI][NfO], coordination of OPPh3 to UO22+ was found, and its coordination number was 4.1 +/- 0.2, indicating UO2(OPPh3)(4)(2+). From the [BMI][NfO] solution containing UO22+ and OPPh3, yellow crystals of UO2(OPPh3)(4)(ClO4)(2) deposited, and its molecular and crystal structures were determined by using single-crystal X-ray analysis. An OPPh3 exchange reaction of UO2(OPPh3)(4)(2+) in [BMI][NfO] was also examined. The apparent first-order rate constant (k(obs)) showed the first-order dependence on [OPPh3] (k(obs) = k(4)[OPPh3](free)), suggesting the "associative" (A) mechanism. Its activation parameters were Delta H-4(double dagger) = 55.3 +/- 2.8 kJ mol(-1) and Delta S-4(double dagger) = 16.1 +/- 7.9 J mol(-1) K-1. To compare the reactivity of UO2(OPPh3)(4)(2+) in [BMI][NfO] with that in an ordinary organic solvent, the same reaction in CD2Cl2 was studied. In the CD2Cl2 system, an equilibrium between UO2(OPPh3)(4)(2+) and UO2(OPPh3)(5)(2+) was observed. The k(obs) values of the OPPh3 exchange reactions in UO2(OPPh3)(4)(2+) (k(4obs)) and UO2(OPPh3)(5)(2+) (k(5obs)) in CD2Cl2 are expressed as k(4obs) = k(4)[OPPh3](free) and k(5obs) = k(5), respectively, indicating that the exchange reactions in UO2(OPPh3)(4)(2+) and UO2(OPPh3)(5)(2+) are categorized in A and "dissociative" (D) mechanisms, respectively. The activation parameters of these reactions were also estimated (UO2(OPPh3)(4)(2+): Delta H-4(double dagger) = 7.1 +/- 0.3 kJ mol(-1) and Delta S-4(double dagger) = -122 +/- 1 J mol(-1) K-1, UO2(OPPh3)(5)(2+): Delta H-5(double dagger) = 62.4 +/- 1.0 kJ mol(-1) and Delta S-5(double dagger) = 68.4 +/- 4.2 J mol(-1) K-1). A large difference in the reactivity of UO2(OPPh3)(4)(2+) was found between [BMI][NfO] and CD2Cl2. This was explained by the formation of a specific solvation barrier of NfO(-) around UO2(OPPh3)(4)(2+) in [BMI[NfO].