Uranium versus Thorium: Synthesis and Reactivity of [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)U[η (2)‐C(2)Ph(2)]

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)U=P‐2,4,6‐tBu(3)C(6)H(2) (1) yields the stable uranium metallacyclopropene, [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)U[η (2)‐C(2)Ph(2)] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U‐(η (2)‐C=C) moiety increases significantly compared to the related Th(IV) compound [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)Th[η (2)‐C(2)Ph(2)], which also results in more covalent bonds between the [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)U(2+) and [η (2)‐C(2)Ph(2)](2−) fragments. Although the thorium and uranium complexes are structurally closely related, different reaction patterns are therefore observed. For example, 2 reacts as a masked synthon for the low‐valent uranium(II) metallocene [η (5)‐1,2,4‐(Me(3)C)(3)C(5)H(2)](2)U(II) when reacted with Ph(2)E(2) (E=S, Se), alkynes and a variety of hetero‐unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five‐membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.