The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (eta(5)-C5Me5)(2)UCl2 (1) with potassium graphite :(KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC=CSiMe3) allows the first stable uranium metallacyclopropene (eta(5)-C5Me5)(2)U[eta(2)-C-2(SiMe3)(2)] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(N) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(eta(2)-C=C) Moiety, leading to more covalent bonds between the (eta(5)-C3Me5)(2)U2+ and [eta(2)-C-2(SiMe3)(2)](2-) fragments than those in the related Th(IV) compound. Consequently, very different reactivity pattertis emerge, e.g., 2,can act as a source for the (eta(5)-C5Me5)(2)U(II) fragment when reacted with alkynes and a :variety of heterounsatutated molediles such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.