We report uranium(IV)-carbene-imido complexes
[U(BIPMTMS)(NCPh3)(κ2-N,N'-BIPY)] (2) (BIPMTMS =
C(PPh2NSiMe3)2; BIPY = 2,2-bipyridine) and
[U(BIPMTMS)(NCPh3)(DMAP)2] (3) (DMAP = 4-dimethylaminopyridine)
that contain unprecedented, discrete R2C=U=NR' units.
These complexes complete the family of E=U=E (E = CR2, 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-/transisomerisation
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 transinfluence
from steric contributions - to be more stable than the
trans-isomer by ~12 kJ mol-1 with an isomerisation barrier of ~14