| Sumario: | The synthesis, structure, and reactivity of an actinide metallacyclopropene were comprehensively studied. The reduction of [η-1,2,4-(MeC)CH]ThCl (1) with potassium graphite (KC) in the presence of diphenylacetylene (PhC ≡ CPh) yields the first stable actinide metallacyclopropene [η-1,2,4-(MeC)CH]Th (η²-CPh) (2). The magnetic susceptibility data show that 2 is indeed a diamagnetic Th(IV) complex, and density functional theory (DFT) studies suggest that the 5f orbitals contribute to the bonding of the metallacyclopropene Th--(η²-C=C) moiety. Complex 2 shows no reactivity toward alkynes, but it reacts with a variety of heterounsaturated molecules such as aldehyde, ketone, carbodiimide, nitrile, organic azide, and diazoalkane derivatives. DFT studies complement the experimental observations and provide additional insights. Furthermore, a comparison between Th and group 4 metals reveals that Th shows unique reactivity patterns.
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