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

C-13 NMR spectra for a series of dinuclear acetate-bridged cyclopalladated N-methyl-N-nitrosoanilines and several mononuclear ortho-substituted pallad ium-aryl complexes with the chelating nitrogen ligands TMEDA and bipyridine were recorded. The coordination chemical shifts for the para carbons in th e Pd-aryl ring, Delta delta C4 (= delta C4 complex -delta C4 ligand), are f ound to be ca --3 to --6 ppm. Despite the possibility that these low-freque ncy shifts might arise from delocalization of electrons from the palladium( II), it is suggested that these observations are not directly related to in creased electron density at this carbon center. Copyright (C) 2000 John Wil ey & Sons, Ltd.