Why does the disorder of R-pn and rac-pn ligands in the quasi-one-dimensional bromo-bridged NiIII complexes, [Ni(pn)2Br]Br2 (pn=1,2-diaminopropane) afford similar STM patterns?

The disordered patterns of R- and rac-1,2-diaminopropane (pn) in quasi-one-dimensional bromo-bridged Ni(III) complexes, [Ni-III(pn)(2)Br]Br-2, have been investigated by single-crystal X-ray structure determination and scanning tunneling microscopy (STM). X-ray structure determination shows that the methyl moieties are disordered on the right- and left-hand sides with half occupancies in both compounds, while the carbon atoms of the ethylene moieties of pn ligands are disordered in [Ni(rac-pn)(2)Br]Br-2 and not disordered in [Ni(R-pn)(2)Br]Br-2. In the STM images of both compounds, the bright spots are not straight but fluctuated with the similar patterns. We have concluded that tunnel current from the STM tip to metal ions are detected via methyl groups of pn ligands.