Addition of the Wittig reagent Ph(3)P=CH(2) to the U(III) tris(amide) U(NR(2))(3) (R = SiMe(3)) generates a mixture of products from which the U(IV) complex U=CHPPh(3)(NR(2))(3) (2) can be obtained. Complex 2 features a short U=C bond and represents a rare example of a uranium carbene. In solution, 2 exists in equilibrium with the U(IV) metallacycle U(CH(2)SiMe(2)NR)(NR(2))(2) and free Ph(3)P=CH(2). Measurement of this equilibrium as a function of temperature provides Delta H(rxn) = 11 kcal/mol and Delta S(rxn) = 31 eu. Additionally, the electronic structure of the U=C bond was investigated using DFT analysis.