1.    Intramoleular electronic coupling at the mixed-valent state:

Designing of polynuclear metal complexes which can exhibit strong intermetallic electronic coupling across the bridge in the mixed valent state (Scheme 1) is a formidable challenge. The interest in these compounds was originally fostered by their function as test systems for experimental and theoretical studies of electron transfer kinetics. Later,


the focus has been shifted to the application potential of mixed-valent systems for molecular electronics (wires, switches, quantum cellular automata, molecular computing). In this regard, a wide variety of mixed valent poly-ruthenium and osmium complexes have been designed using selective bridging and ancillary functionalities. Our systematic approaches beyond the conventional systems by using sophisticated ligands and by creating more variegated coordination patterns have demonstrated the following fundamental points: (i) In addition to the effect of the spacer groups (bridging functionalities), the electronic aspects of the ancillary functions can also contribute significantly towards the intermetallic coupling processes. (ii) Large electrochemical coupling can be independent of the intensity of the intervalence charge transfer transition (IVCT) in the mixed valent state. (iii) Relatively smaller electrochemical coupling can also be associated with the intense and narrow intervalence transition like a delocalized valence situation due to the involvement hybrid situation. (iv) Recognition and structural/spectroscopic characterization of rather elusive d4/d5 (RuIV/RuIII) ions derived mixed valent set up. (v) The emergence of a fascinating coordination chemistry with non-innocent bridging/ancilliary ligands has raised the danger of mistaking radical complexes for mixed-valent intermediates, which could however be differentiated via collective consideration of sophisticated experimental techniques along with theoretical deliberation.

Representative References