STM Imaging of Three-Metal-Center Molecules: Comparison of Experiment and Theory For Two Mixed-Valence Oxidation States

The molecule {Cp*(dppe)Fe(C≡C−)}3(1,3,5-C6H3) (Fe3) was adsorbed on a single-crystal gold surface and studied using ultrahigh-vacuum scanning tunneling microscopy (STM). Both the singly oxidized Fe3+ and doubly oxidized Fe32+ are mixed-valence ions, and localization of the charge at specific metal centers was observed as the appearance of pronounced asymmetry in STM images. Switching the tip–sample bias voltage demonstrates that this asymmetry is electronic in nature. The nature of intramolecular structure and the degree of asymmetry produced in STM images varies according to the state of the scanning tip. Constrained density functional theory was used to simulate STM images for the neutral molecule and for both mixed-valence species, and simulated images agreed closely with observed results. In particular, changing the number of molecular electronic states contributing to contrast in the STM image produced a good match to the variation in structures measured experimentally.