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Tuning the Electronic Communication in Heterobimetallic Mixed-Valence Ions of (1-Ferrocenyl)- and (2-Ferrocenyl)indenyl Rhodium Isomers



A series of heterobimetallic complexes of general structure [RhL25-(2-ferrocenyl)indenyl}] (L2=cod, nbd, L=CO; cod=cyclooctadiene; nbd=norbornadiene) has been synthesised with the aim of tuning the metal–metal interaction in their mixed-valence ions generated both by chemical and electrochemical oxidation, and the results are compared with those obtained for [RhL25-(1-ferrocenyl)indenyl}] isomers. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes evidencing a significant difference in the extent of planarity of the flexible bridging ligand between the 1- and 2-ferrocenyl isomers. Independent experimental probes, in particular the potential splitting in the cyclic voltammograms and the IT bands in the near-IR spectra, are rationalised in the framework of Marcus–Hush theory and at quantum chemistry level by DFT and TD-DFT methods. These methods allow us to establish a trend based on the magnitude of iron–rhodium electronic coupling Hab ranging from valence trapped to almost delocalised ions. The quasi planar bridge and the olefin ancillary ligands make [Rh(nbd){η5-(2-ferrocenyl)indenyl}]+ and [Rh(cod){η5-(2-ferrocenyl)indenyl}]+ rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.