The uranium metallacyclocumulene, [η⁵-1,3-(Me₃C)₂C₅H₃]₂U(η⁴-C₄Ph₂) (2) was isolated by the reduction of [η⁵-1,3-(Me₃C)₂C₅H₃]₂UCl₂ (1) with potassium graphite (KC₈) in the presence of 1,4-diphenylbutadiyne (PhCC–CCPh) in good yield. Furthermore it was fully characterized including the determination of its molecular structure; and the reactivity of 2 towards various small unsaturated organic molecules was explored. For example, while complex 2 shows no reactivity with alkynes and 2,2′-bipyridine (bipy), it reacts as a nucleophile when exposed to carbodiimides, diazabutadienes, isothiocyanates, ketones, and pyridine derivatives, leading to five-, seven- or nine-membered heterometallacycles. In contrast, treatment of complex 2 with CS₂ results in CS bond cleavage and forms the binuclear complex [η⁵-1,3-(Me₃C)₂C₅H₃]₂U[μ-η⁴:η³-PhCCC(S)C(Ph)CS]U[η⁵-1,3-(Me₃C)₂C₅H₃]₂ (10). Density functional theory (DFT) studies complement the experimental study.