  Coordination chemistry relies on harnessing active metal sites within organic
matrices. Polynuclear complexes - consisting of organic ligands binding to
clusters of several metal atoms are of particular interest, owing to their
electronic/magnetic properties and potential for functional reactivity
pathways. However, their synthesis remains challenging; only a limited number
of geometries and configurations have been achieved. Here, we synthesise - via
supramolecular chemistry on a noble metal surface - one-dimensional
metal-organic nanostructures composed of terpyridine (tpy)-based molecules
coordinated with well-defined polynuclear iron clusters. By a combination of
low-temperature scanning probe microscopy techniques and density functional
theory, we demonstrate that the coordination motif consists of coplanar tpy's
linked via a linear tri-iron node in a mixed (positive) valence, metal-metal
bond configuration. This unusual linkage is stabilized by a local accumulation
of electrons at the interface between cations, ligand and surface. The latter,
enabled by the bottom-up on-surface synthesis, hints at a chemically active
metal centre, and opens the door to the engineering of nanomaterials with novel
catalytic and magnetic functionalities.
