ELECTRON DELOCALIZATION IN PARAMAGNETIC METALLOCENCES .I. NUCLEAR MAGNETIC RESONANCE CONTACT SHIFTS

The 1,1 ′-dimethylcyclopentadienyls of V, Cr, Co, and Ni were prepared, and the nmr contact shifts were determined at 60 MHz in toluene. Analysis of electron relaxation times and nmr line widths suggests that the dominant nuclear relaxation mechanism is dipolar. The temperature dependencies of the contact shifts were examined, and it was shown that the Curie law is obeyed in each case. It was shown that the pseudocontact (dipolar) shift should be negligible in all cases except perhaps at the methyl group in 1,1 ′-dimethylchromocene. Comparison of the methyl contact shifts and the ring contact shifts leads to the conclusion that the unpaired electron density in vanadium and chromium metallocenes is not in ring π orbitals (out of plane) as previously supposed. Instead the magnitudes and signs of the chromium and vanadium shifts strongly suggest that the unpaired spins are delocalized in σ (in-plane) cyclopentadienyl orbitals. Delocalization in π-type orbitals is found to predominate in cobalt and nickel metallocenes. © 1969, American Chemical Society. All rights reserved.