# Experimental and Theoretical Studies of Isomeric Metal (N^C^N)Cl Coordination Complexes (Metal = Pt, Pd) with Multiple Conductance Pathways in Single-Molecule Junctions

## TL;DR

This study explores how the structure of metal complexes affects their electrical conductivity in single-molecule junctions.

## Contribution

The research reveals that metal coordination and orbital mixing influence conductance more than quantum interference in these systems.

## Key findings

- Metal coordination increases conductance compared to free ligands in single-molecule junctions.
- Complexes with meta-substituted ligands show higher conductance than their para-analogs.
- Orbital mixing between metal centers and ligand π-orbitals plays a greater role than quantum interference.

## Abstract

The present work provides insight into the effect of connectivity
within isomeric 3,5-bis­(pyridin-2-yl)­phenyl (N^C^N) 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.