Imaging an unsupported metal-metal bond in dirhenium molecules at the atomic scale

Metallic bonds remain one of the most important and least understood of the chemical bonds. In this study, we generated Re-2 molecules in which the Re-Re core is unsupported by ligands. Real-time imaging of the atomic-scale dynamics of Re-2 adsorbed on a graphitic lattice allows direct measurement of Re-Re bond lengths for individual molecules that changes in discrete steps correlating with bond order from one to four. Direct imaging of the Re-Re bond breaking process reveals a new bonding state with the bond order less than one and a high-amplitude vibrational stretch, preceding the bond dissociation. The methodology, based on aberration-corrected transmission electron microscopy imaging, is shown to be a powerful analytical tool for the investigation of dynamics of metallic bonding at the atomic level.