The present work provides insight into the effect of connectivity within isomeric 3, 5-bis- (pyridin-2-yl) -phenyl (NCN) platinum and palladium complexes on their electron transmission properties within gold|molecule|gold junctions. The ligands 3, 5-bis- (4- (methylthio) -pyridin-2-yl) -phenyl hexanoate (L m H) and 3, 5-bis- (5- (methylthio) -pyridin-2-yl) -phenyl hexanoate (L p H) were synthesized and coordinated with either PtCl or PdCl to form complexes Pt m, Pt p, Pd m and Pd p. X-ray photoelectron spectroscopy (XPS) measurements evaluated the contacting modes of the molecules in the junctions. A combination of scanning tunneling microscopy-break junction (STM-BJ) measurements and density functional theory (DFT) calculations demonstrate that for the single-molecule S···S contacted junctions metal coordination enhanced the conductance compared with the free ligands. Notably, the higher degree of orbital mixing between the metal center and the ligand π-orbitals in the metal complexes plays a greater role than quantum interference to the extent that the complexes that incorporate ligands substituted with thiomethyl groups in meta positions relative to the pyridine-benzene linkages have a higher conductance than their para-analogs, e. g. , Pt p -3. 8 log- (G/G 0) and Pt m -3. 3 log- (G/G 0), in contrast to the usual conductance trend (para > meta) for purely organic π-electron systems.
Bastante et al. (Fri,) studied this question.