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.