Organometallic radicals of iron and ruthenium: Similarities and dissimilarities of radical reactivity and charge delocalization

Abstract

Abstract Redox chemistry of d6 iron(II) and ruthenium(II) complexes has been studied extensively due to the interesting reactivity of mononuclear radical species and the charge-delocalized properties of dinuclear mixed-valence (MV) complexes. In particular, organometallic dinuclear MV systems show long-range charge-delocalized behavior, and their properties can be modified by the choice of the metal (M) and the bridging ligand (BL). Here we review mono- and di-nuclear radical species having CP2M and CPM(dppe) fragments (CP\u202f=\u202fη5-C5H5 or η5-C5Me5; M\u202f=\u202fFe, Ru) as the redox centers. Although the Fe and Ru species are frequently isostructural, the electronic structures of them are considerably different. Radical center tends to localize rather on the metal center and the ligand part in the Fe and Ru complexes, respectively. For mononuclear species, the Ru complexes exhibit reactivity at the ligand to undergo radical coupling, whereas the Fe complexes may stay as monomeric radical species. For dinuclear complexes, the radical center of mixed-valence diiron species rather localizes on the metal center, while that of the diruthenium analogues tends to localize on the BL. The difference could be ascribed to reorganization caused by the electron transfer, and the extent of the contribution of the BL-localized state can be estimated by the difference between the νmax values for an Fe complex and its Ru derivative (Δνmax(Ru-Fe)).