Abstract/Summary

In this chapter, we have outlined a range of symmetrical oligonuclear Group 6–9 metal complexes with redox-active metal–ethynyl termini linked directly to each other or to a core unit of the bridge represented by diverse conjugated, saturated, or ligated metallic groups. The transition metals in the terminal parts of the molecular chains include chromium, molybdenum, manganese, rhenium, iron, ruthenium, osmium, and cobalt in diverse oxidation states, forming redox series. Their electronic coupling properties in the MV states have been assessed. The degree of electronic communication between the metal–ethynyl centers in the MV systems can be evaluated on the grounds of electrochemical data, IR, UV–vis–NIR spectroelectrochemical monitoring, and EPR spectra in combination with theoretical calculations. The MV characteristics strongly depend on the structural properties of the bridging ligands, such as the length and coplanarity, and the degree of their conjugation. The interplay between the bridge properties and the nature of the linked metal centers, the stability and localization of the oxidation states, and the strength of the coupling between the metal centers and the ancillary ligands also effectively affects the electron transfer processes. Special attention needs to be paid to the analysis of electronic absorption of the open-shell bimetallic complexes in the NIR and short-wave infrared (SWIR) region. Apart from the characteristic IVCT absorption attributed to the thermal population of MV states (Class II and III, and the borderline between them), the coexistence of bridge-localized oxidation (consistent with a high degree of C≡C–core–C≡C π-characteristic in the frontier orbitals and a redox non-innocent characteristic) introduces low-lying π–π* (intra-bridge) or MLCT electronic transitions in this region. Their simultaneous population may be detected in configurational rotamers where the plane of an diethynyl aromatic core of the bridge varies its orientation with respect to the metal d-orbitals of appropriate π-symmetry. To sum up, the family of homodinuclear metal–ethynyl complexes offers a great potential in exploring thermal or photo-induced intramolecular electron transfer properties. However, despite the rigid structure of the metal–ethynyl units, the synthesis of extended MV molecular wire models with remote charge transfer ability remains a challenge.