Uranium–Carbene–Imido Metalla‐Allenes: Ancillary‐Ligand‐Controlled cis ‐/ trans ‐Isomerisation and Assessment of trans Influence in the R 2 C=U IV =NR′ Unit (R=Ph 2 PNSiMe 3 ; R′=CPh 3 )

Uranium(IV)–carbene–imido complexes [U(BIPM TMS )(NCPh 3 )(κ 2 ‐ N , N ′‐BIPY)] ( 2 ; BIPM TMS =C(PPh 2 NSiMe 3 ) 2 ; BIPY=2,2‐bipyridine) and [U(BIPM TMS )(NCPh 3 )(DMAP) 2 ] ( 3 ; DMAP=4‐dimethylamino‐pyridine) that contain unprecedented, discrete R 2 C=U=NR′ units are reported. These complexes complete the family of E=U=E (E=CR 2 , NR, O) metalla‐allenes with feasible first‐row hetero‐element combinations. Intriguingly, 2 and 3 contain cis ‐ and trans ‐C=U=N units, respectively, representing rare examples of controllable cis / trans isomerisation in f‐block chemistry. This work reveals a clear‐cut example of the trans influence in a mid‐valent uranium system, and thus a strong preference for the cis isomer, which is computed in a co‐ligand‐free truncated model—to isolate the electronic trans influence from steric contributions—to be more stable than the trans isomer by approximately 12 kJ mol −1 with an isomerisation barrier of approximately 14 kJ mol −1 .