Spin-Density Distributions in Conjugated Ligands of Paramagnetic Chelates from NMR Contact Interaction Shifts

Electron-spin-density distributions in a variety of π systems have been obtained from proton-contact interaction shifts in paramagnetic nickel II chelates of N,N′-disubstituted aminotroponeimines. The results have furnished a great deal of new information regarding the detailed manner in which π electrons are distributed via conjugative effects. Spin-density distributions in these systems calculated by the valence-bond scheme are shown to be in quite good general agreement with experimental determinations. Effects attributable to hyperconjugation have been observed and the conjugating abilities of linking groups C=C, N=N, NH, O, S, and SO2 have been assessed. It is shown also that metal-ligand π-bond formation results in transfer of 0.1 of an unpaired electron from nickel to the π system of each of the ligands of these bischelates.