Bridge Effect in Metallocomplex Ion-Radicals

Lloveras et al. (2005) compared intervalence electron transfer in l,4-diferrocenyl-l,3-butadiene, [Fc—CH=CH—CH=CH—Fc] andin l,4-diferrocenyl-2-aza-l,3-butadiene, [Fc—CH=N— CH=CH—Fc]. Although intervalence electron transfer in [Fc—CH=CH—CH=CH—Fc] and [Fc—CH=CH—Fc] resemble each other, this process in [Fc—CH=N—CH=CH—Fc] is restricted to some degree. As the authors noted, the population of the state with the positive charge located on the iron attached to the 4-position of the 2-aza-l,3-butadiene bridge is dominant, with just a very small population ( 0.1%) of the state with the charge on the other iron site (Lloveras et al. 2005). 

Signihcant electronic interaction between the iron centers was also found in the cation-radicals of diferrocenylthiazoles (Tarraga et al. 2002, Plazuk et al. 2005) and in the cation-radical of Fc(Ph)C=C=C=C=C=C(Ph)Fc cumulene molecular wire (Skibar et al. 2004). In the latter cation-radical, an unpaired electron (a hole) is half-to-half localized on each Fe atom. No apparent spin density was found on the cumulene chain (Bildstein et al. 2004). 

An interesting picture was revealed by Gouverd et al. (2006) as a result of comparison between the neutral chromium tricarbonyl complex of C,P-diaryl phosphaalkene [(CO)3Cr —PhCH=PMes ] and its anion-radical [(CO)3Cr — PhCH=PMes ] (Mes is 2,4,6-tris(tert-butyl)phenyl). According to the ESR and DFT data for the anion-radical, about 40% of the negative charge is localized on Cr(CO)3 moiety. This is understandable if one takes into account the known electron-acceptor 

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