Oxidations by vanadium IV

A V(III)-Cr(in) dimer has been detected as an intermediate in the V(IV) + Cr(II) reaction and its formation and subsequent decomposition studied. Since V(IV) was arranged to be in excess of Cr , no complications ensued from the latter ion bringing about further reduction of V(III) to V(II). Under these conditions the overall reaction corresponds accurately to 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

The one-equivalent oxidation of Eu(Il) by V(IV) takes place rapidly at 0 °C, 0.2 M and 10 M reactant concentrations, the second-order rate coeffi- 

Pig- 1. Absorption spectrum of V(III)-Cr(III) dimer formed as an intermediate in the V(IV) 4-Cr(II) reaction (the ordinate is directly proportional to the absorbance) 0.200 M HCIO4 temp., 15.0 C. From Espenson, by courtesy of The American Chemical Society.) 

Mode of formation Binuclear species Specific decomposition rate at 25.0 °C (sec ) log k for formation of SCN complex Ref 

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