The mixed-valence binuclear complexes,
[(NC)5Fe(pyCN)Ru(NH3)5]
n
(4- and 3-cyanopyridine isomers, with
nitrile-N and pyridine-N binding to Ru and Fe,
respectively) were prepared as solid compounds through
stoichiometric oxidation of the fully reduced (II,II) binuclear
complexes, R, with peroxydisulfate. By analysis of
IR spectra, the solids were observed to be a mixture of the predominant
electronic isomers with a FeII,
RuIII
distribution, with minor amounts of the FeIII,
RuII isomers. In aqueous solution, R was oxidized with
peroxydisulfate
to M, the mixed-valence complex, and to Ox, the fully oxidized complex.
The M complex shows an intervalence
band at 938 nm; by application of the Hush model, it is described as a
valence-trapped RuII, FeIII complex;
the
latter electronic distribution is supported by UV−visible,
electrochemical, and kinetic data, but a minor amount
of the isomer with a FeII, RuIII distribution
is also present in the equilibrium. The M complex is unstable
toward
dissociation and further outer-sphere reactions, leading to hydrolyzed
products in the time scale of minutes.
Hydrolysis is also the main decomposition route of the Ox complex.
In the reactions with excess peroxydisulfate,
the analysis of successive spectra allows the elucidation of the rate
constants for the one-electron processes leading
to M and Ox. The rate constants for the formation and dissociation
of M, as well as for the hydrolysis of Ox,
were also obtained. A kinetic control is operative in the
oxidation reactions, with a preferential attack of
peroxydisulfate on the more reactive Ru(II) center. The role
of electronic isomerization is discussed in the overall
kinetic scheme, and the rate constant values for oxidation agree with
predictions based on Marcus LFER, in
accord with data published for related complexes.