The reaction of the sterically saturated uranium(III) tetrasilylamido complex [K(18c6)][U(N(SiMe3)(2))(4)] with CO2 leads to CO2 insertion into the U-N bond affording the stable U(IV) isocyanate complex [K(18c6)][U(N(SiMe3)(2))(3)(NCO)(2)](n) that was crystallographically characterized. DFT studies indicate that the reaction involves the [2+2] cyclo-addition of a double bond of O=CO to the U-N(SiMe3)(2) bond and proceeds to the final product through multiple silyl migration steps.