Electronic structure of cyano-bridged dinuclear iron complexes

A series of complexes of formula [(NC)(5)Fe-II-NC-Fe-II(CN)(4)L](n-), with L = H2O, pyridine, isonicotinamide and 4-cyanopyridine were prepared in aqu eous solution by substitution of the corresponding [Fe-II(CN)(5)L](n-) ions into [Fe-II(CN)(5)H2O](3-). The mixed valent (II, III) and fully oxidized (III, III) complexes were also obtained. The (II,II) complexes were moderat ely stable toward dissociation into the mononuclear species, but the mixed- valent ions were properly characterized by UV-vis-NIR spectroscopy and elec trochemistry. Distinctive intervalence (IV) bands were assigned in the NIR region, with the energy being dependent on the binding properties of L; the IV band energy also correlated with the redox potential at the [NC-Fe(CN)( 4)L] fragment. By application of the Hush model, a valence-trapped situatio n was found for the [(NC)(5)Fe-III -NC-Fe-II(CN)(4)L](n-) ions. The class I I behavior shows, however, a value of H-ab, the electronic coupling factor, of ca. 1600 cm(-1), indicating a moderate-to-strong communication between the metal centers.