SLOWED TRIPODAL ROTATION IN ARENE CHROMIUM COMPLEXES - STERIC AND ELECTRONIC BARRIERS

The syntheses of bis(arene)chromium and (arene)Cr(C0)3complexes (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)2Cr 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)3tripod was spinning freely on the NMR time scale. © 1992 Academic Press Inc.