13C NMR of Pd-aryl complexes. Chemical shifts and Pd π-back-bonding

13C NMR spectra for a series of dinuclear acetate-bridged cyclopalladated N-methyl-N-nitrosoanilines and several mononuclear ortho-substituted palladium–aryl complexes with the chelating nitrogen ligands TMEDA and bipyridine were recorded. The coordination chemical shifts for the para carbons in the Pd–aryl ring, Δδ C4 (= δ C4 complex −δ C4 ligand), are found to be ca −3 to −6 ppm. Despite the possibility that these low-frequency shifts might arise from delocalization of electrons from the palladium(II), it is suggested that these observations are not directly related to increased electron density at this carbon center.