Slowed Tripodal Rotation in Arene-Chromium Complexes: Steric and Electronic Barriers

Publisher The syntheses of bis(arene)chromium and (arene)Cr(C0)3 complexes (1–3) naturally prompted questions as to the mode of interaction of the metal with the aromatic ring system. An early X-ray crystallographic study on (C6H6)2 Cr suggested an alternating pattern of carbon–carbon bond lengths. However, subsequent determinations did not support this view. The initial nuclear magnetic resonance (NMR) data on the half-sandwich molecules revealed that the proton chemical shifts in π-complexed rings are considerably shielded relative to their resonance positions in the free arenes. Subsequently, attention focused on whether the observed chemical shifts reflected the presence of the single rotamer found crystallographically in the solid state or whether the M(CO)3, tripod was spinning freely on the NMR time scale.