Direct observation of 57Fe chemical shifts in diamagnetic organoiron complexes

⁵⁷Fe NMR spectra of 35 organoiron complexes predominantly of the type [Fe(CO)₃(diene)] have been measured by direct detection at 2.9 MHz and at natural isotope abundance (2.2%). The ⁵⁷Fe chemical shift range so far observed is 3000 ppm, corresponding to about 9 KHz. The majority of the resonances are at higher frequency than that of Fe(CO)₅, which is proposed as a secondary standard. The ⁵⁷Fe resonance frequencies are also reported as ratios relative to the ⁷³Ge frequency of GeCl₄.The ⁵⁷Fe shielding is discussed qualitatively in terms of charge distribution in the complexes, very large deshielding effects being observed for cationic olefinic ligands. For η⁴-diene complexes, the shielding also depends upon ligand geometry and decreases with increasing ring size or CCC bond angle. Two types of ferrocenylcarbenium ions may be distinguished, indicated by high- or low-frequency shifts relative to ferrocene. For a number of [Fe(CO)₃(olefin)] complexes ¹J(⁵⁷Fe, ¹³C) coupling constants involving the carbonyl carbon are also reported.