The cyanide ligand as an efficient bridge in mixed-valence complexes

Abstract

The mixed-valence complexes [Cp(dppe)Fe-(μ-CN)-Fe(dppe)Cp](PF6)2 (1+) and [Cp(dppe)Fe-(μ-CN)-Mn(CO)2(dppm)-P(OPh)3] (PF6)2 (2+) generated by oxidation of their respective monocationic precursor 1 and 2, exhibit moderately intense intervalence transition (IT) at 880 nm for 1+ and at 971 nm for 2+. The application of the Hush theory to these IT indicates a considerable interaction between the metal centers which behave as Class II Robin-Day (α = 0.12, Hab = 1369cm−1 for 1+; α = 0.09, Hab = 1198cm−1 for 2+). The metal-metal interaction for a series of cyanide bridged mixed-valence complexes exhibits a characteristic Hab value in the range 1000–1700 cm−1 indicating a strong interaction. A free-energy, cross linear relation between in kth (the rate electron transfer kth estimated using the Hush model) and the difference in redox potential ΔE0 of the two metalic centers was found for several cyanide and other bridged bimetallic complexes. Plots of [Eop−ΔE0] (Eop the energy of the IT band at λmax) versus [(1/Dop) − (1/Ds)] have been made for 1+ and 2+ and the linear relationships predicted by the Hush theory have been found. The reorganizational outer-sphere E0 and inner-shape E1 parameters were determined and discussed.A comparison of these results with those for other mixed-valence complexes containing cyanide or other bridged ligands suggests that the cyanide ligand is an effective bridge between the metallic fragments.