Square-Planar–Tetrahedral Interconversion without Spin Flip in (β-diiminate)Rh(1,3-diene) Complexes

Rhodium β-diiminate complexes of conjugated dienes, (RBDI)­Rh­(diene) (RBDI = [2,6-R2C6H3-NCMe]2CH; R = Me, Et; diene = 1,3-hexadiene, 1,3-pentadiene, 1,3-butadiene, 2-Me3SiO-1,3-cyclohexadiene) as well as (MeBDI)­Ir­(1,3-butadiene) show fluxional behavior caused by square-planar–tetrahedral interconversion where the diene rotates relative to the diiminate ligand. The mechanism involves neither (partial) ligand dissociation nor a change in spin state. Density functional calculations support the rotation mechanism and indicate that it is assisted by a partial valence isomerization of the diene to an enediyl ligand at the tetrahedral geometry. Nonconjugated dienes, which cannot undergo such valence isomerization, are predicted to have much larger barriers to rotation.