THE CYANIDE LIGAND AS AN EFFICIENT BRIDGE IN MIXED-VALENCE COMPLEXES

The mixed-valence complexes [Cp(dppe)Fe-(mu-CN)-Fe(dppe)Cp](PF6)(2) (1 (+)) and [Cp(dppe)Fe-(mu-CN)-Mn(CO)(2)(dppm) P(OPh)(3)](PF6)(2) (2(+)) generated by oxidation of their respective monocationic precursor 1 a nd 2, exhibit moderately intense intervalence transition (IT) at 880 n m for 1(+) and at 971 nm for 2(+), The application of the Hush theory to these IT indicates a considerable interaction between the metal cen ters which behave as Class II Robin-Day (alpha = 0.12, H-ab = 1369 cm( -1) for 1(+); alpha = 0.09, H-ab = 1198 cm(-1) for 2(+)). The metal-me tal interaction for a series of cyanide bridged mixed-valence complexe s exhibits a characteristic H-ab value in the range 1000-1700 cm(-1) i ndicating a strong interaction. A free-energy, cross linear relation b etween In k(th) (the rate electron transfer k(th) estimated using the Hush model) and the difference in redox potential Delta E-0 of the two metalic centers was found for several cyanide and other bridged bimet allic complexes, Plots of [E-op - Delta E-0] (E-op the energy of the I T band at lambda(max)) versus [(1/D-op) - (1/D-s)] have been made for 1(+) and 2(+) and the linear relationships predicted by the Hush theor y have been found. The reorganizational outer-sphere E-o and inner-sph ere E-i parameters were determined and discussed. A comparison of thes e results with those for other mixed-valence complexes containing cyan ide or other bridged ligands suggests that the cyanide ligand is an ef fective bridge between the metallic fragments. (C) 1998 Elsevier Scien ce S.A.