A study is reported on the relative stability of trivalent bis(ligand) complexes of the form [M(L(R))(2)N''] for trivalent group 3, lanthanide and actinide cations, using the sterically demanding N-heterocyclic carbene ligand L(R) = [OCMe(2)CH(2){CNCH(2)CH(2)NR}] (R = (i)Pr L(P), Mes L(M), Dipp L(D); N'' = N(SiMe(3))(2)). For the small Y(III) cation (r(6-coord) = 1.040 A) and the smallest L(R), R = (i)Pr, mono, bis, and tris(L(P)) complexes can be made; [Y(L(P))(2)N''] and [Y(L(P))(3)] have been characterised. For the larger ligands, L(M) and L(D), only the mono(L(R)) complexes [Y(L(M))N''(2)] and [Y(L(D))N''(2)] can be made. For the larger Ce(III) (r(6-coord) = 1.15 A), mono(L(R)) and bis(L(R)) complexes [Ce(L(M))N''(2)], [Ce(L(D))N''(2)], [Ce(L(M))(2)N''], and [Ce(L(D))(2)N''] can be made; structural characterisation of the latter two confirm the high degree of steric congestion. The new complex [U(L(M))N''(2)] has also been isolated. Despite the very similar radii of Ce(III) and U(III) (r(6-coord) = 1.165 A), the complexes [U(L(R))(2)N''] cannot be isolated; a surprising display of the difference between the 4f and 5f metal series. However, the six-coordinate, bis(ligand) U(IV) complexes can readily be isolated if smaller ancillary ligands are used; [U(L(M))(2)I(2)] and [U(L(D))(2)I(2)] have been fully, including structurally, characterised.