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 [RhL2-{h5-(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 mixedvalence ions generated both by chemical and electrochemical oxidation,and the results are compared with those obtained for [RhL2{h5-(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)-{h5-(2-ferrocenyl)indenyl}]+ and [Rh- (cod){h5-(2-ferrocenyl)indenyl}]+ rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.