Innocent or guilty? Redox activity in and magnetic and optical behaviour of dinuclear molydenum complexes

The phenomenon of ‘non-innocence’, first articulated by Jørgensen in 1966, is briefly reviewed. Spectroelectrochemical studies of a range of dinuclear complexes of the type [{Mo(NO)Tp*Cl}2(bridge)] (bridge = dipyridyls) and [{Mo(O)Tp*Cl}2(bridge)] (bridge = diphenolates) which are redox active, show that oxidised or reduced forms of these species exhibit ‘non-innocence’. The spectral behaviour is associated with metal-to-ligand or ligand-to-metal charge transfer phenomena, probably the first time that monodentate bridging ligands have been implicated in ‘non-innocent’ behaviour. These bridging ligands also determine the nature and extent of magnetic interaction between the unpaired spins in [{Mo(NO)Tp*Cl}2(bridge)] and [{Mo(O)Tp*Cl}2(bridge)], the dominant mechanism of spin-exchange relying on the extent of ð-delocalisation within the bridging ligands. The unusual optical behaviour of these dinuclear complexes when oxidized (oxomolybdenum diphenolates) or reduced (nitrosyl molybdenum dipyridyls) has led to the exploration of electrochromism as a means to develop variable optical attenuators operating in the near-infrared region.