Molecular orbital calculations on transition metal complexes : XXIII. 1H nmr shifts in sandwich, mixed sandwich, and related tricarbonyl complexes

Abstract

INDO SCF MO calculations have been carried out for a variety of diamagnetic sandwich, mixed sandwich, and related tricarbonyl complexes of the 3 d series, and for the free ligand systems of the cyclopentadienyl anion (Cp −), the neutral benzene molecule (Bz), and the cycloheptatrienyl cation (Ch +). The π bond orders for the CC linkages of the ligand rings all show significant, but broadly comparable, reductions on complexation, and the 1H NMR shift for a given ring proton, relative to that for the appropriate free ligand, Δτ, shows a good linear correlation with the corresponding change in the charge density at that proton, Δ Pππ(H). The plot of Δτ against Δ Pππ(H) shows a positive (upfield) intercept of about 2.5 ppm on the Δτ axis, and it is concluded that the results provide evidence for an appreciable diminution in the aromatic character of the ligand rings on complex formation.