723
chromium(I1) produced in reaction 1 does not significantly affect the rate of disappearance of CrSCN2+. This was further checked by adding Co(NHJ6F2+ to react with the chromium(I1) produced in reaction 1.2°-22 A solution 0.42 M in V2+and 1 M in perchloric acid was mixed with a solution -2 X l e 3 M in Cr~ + , 1 M in SCN2+, 1.2 X le2M in C O ( N H ~ ) ~ Fand perchloric acid, and the formation of VNCS*+ and its subsequent disappearance were followed at 400 mp, an absorption minimum of C O ( N H ~ ) ~ F ~The + . value of f,,,,the time required for the concentration of VNCS2+ to reach its maximum value, was 0.83 sec, in very good agreement with the value of 0.83 sec calculated for reactions 1 and 2.23824We conclude, therefore, that, in contrast to the iron(II1)-vanadium(I1) reactions, the reaction of CrSCN2+ with vanadium(I1) proceeds uia an anion-bridged transition state, in which the thiocyanate is bonded to both the vanadium and the chromium. It is noteworthy that the rate constant for reaction 1 is very similar to the rate constant for the formation of thiocyanatopentaaquovanadium(I1) from vanadium(I1) and thiocyanate i o n ~ . ~ ~ r ~ 6 (20) The rate constants for the reaction of Co(NHa)jF?+ with vanadium(I1) and chromium(I1) are 2.6 and 9 X 106 M-1 sec-1, respectively, at 25.0” and ionic strength 1.0 (21) J. P. Candlin, J. Halpern, and D. L. Trimm, J . Am. Chem. SOC., 86, 1019 (1964). (22) J. P. Candlin and J. Halpern, Inorg, Chem., 4, 766 (1965). (23) A. A. Frost and R. G. Pearson, “Kinetics and Mechanism,” 2nd ed, John Wiley and Sons, Inc., New York, N. Y . , 1961, p 166. (24) G . Friedlander, J. W. I
