Electronic coupling in mixed-valence dinuclear ferrocenes and cobaltocenes with saturated bridging groups

We have synthesised a series of new dinuclear metallocenes [{M(Cp*)(C5H4)}(2)X] (Cp* = eta(5)-pentamethy1cyclopentadienyl; M = Fe, Co, X = CMe2, SiMe2, GcMe(2); M = Fe, X = Si2Me4). For the neutral dicobalt complexes, magnetic susceptibility measurements reveal intramolecular antiferromagnetic interactions of -21 and -14 cm(-1) for SiMe2- and GeMe2-bridged species, respectively, but negligible interaction for the CMe2-bridged compound. In contrast, intervalence charge-transfer (IVCT) data for the mixed-valence monocations of both Fe and Co complexes show electronic coupling to decrease in the order CMe2 > SiMe2 > GeMe2. This suggests that electronic coupling is principally through-space in contrast to results found from previous studies. The IVCT data also show much stronger coupling in the dicobalt species versus their diiron analogues.