Electronic coupling in cyclometalated ruthenium complexes

•Electronic coupling in both symmetric and redox-asymmetric cyclometalated diruthenium complexes is summarized.•Electronic coupling in ruthenium-amine and ruthenium-ferrocene conjugates is analyzed by the two-state Hush theory.•DFT and TDDFT results are used to rationalize the electronic properties and charge transfer transitions of mixed-valence compounds.•Electronic coupling through a NCN-NCN-, NNN-NNN-, or NNC-NNC-type of bridging ligand is compared.

This review summarizes the electronic coupling studies in mixed-valence or donor–acceptor -type complexes containing one or two cyclometalated ruthenium redox sites. Basic electron transfer theories and analysis methods for mixed-valence compounds are introduced, followed by the discussion on the electronic coupling studies of symmetric diruthenium complexes. The effects of the RuC bond and the position of the RuC bond on the degree of electronic coupling are highlighted. The uses of different bridging ligands and terminal ligands can greatly modulate the degree of coupling. Complexes with a [NCN-NCN]-type bridging ligand show much stronger electronic coupling with respect to those with a [NNN-NNN]- or [NNC-NNC]-type bridging ligand. In addition, two types of asymmetric complexes are discussed, including redox-asymmetric diruthenium complexes and monoruthenium complexes containing an additional redox-active site. The two-state Hush theory is used to derive the electronic coupling parameters of these complexes. DFT and TDDFT calculations are used to rationalize the spin density distribution and charge transfer transitions of selected odd-electron complexes. Future aspects of the studies on redox-active cyclometalated ruthenium complexes are briefly discussed.

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