The Exchange Reactions of Chromium(II) Ion and Certain Chromium(III) Complex Ions¹

Some Aspects of Ligand Effects in Electron-Transfer Reactions

“…From the results of Ball and King [7] and the data on the affinities of Cr 3 + for F~ and Cl~, Q c \ fF can be calculated to be about 2 X 10~3. In this outer-sphere activated complex, in which the halide can make a bond only to the reducing agent, the trend is opposite to that observed when the halide can make a bond only to the oxidant.…”

“…When halide complexes are reduced by cationic reducing agents which act by outer-sphere mechanisms, in every system encountered thus far, the rate of reduction increases as the size of the halide increases [5,4]. The data for Cr 2+ were selected because of the 2.6x10-» 9 > 6 0 - [7] 2 . The data for Cr 2+ were selected because of the 2.6x10-» 9 > 6 0 - [7] 2 .…”

Reduction of some acidopentaaquochromium(III) complexes at the mercury electrode

“…Homogeneous oxidation-reduction reactions in the double layer may also play a role. Although the reduction of CrNCS 2+ and CrF + is slow and of CrCl 2+ is fast [14], the reaction rates in solutions containing excess free ligand are not known. By such a reaction a complex which is faster or slower reduced at the mercury electrode can be formed.…”

“…Therefore it is possible that CrNCS 2÷ is reduced by a mechanism in which the NCS-ligand is a bridge for the electron transfer between chromium(liD ion and the electrode, analogous to the ligand bridging mechanism in the electrochemical oxidation of chromium(lI) at mercury electrodes in the presence of (pseudo) halides and to the inner sphere mechanism of homogeneous reduction [14]. The same mechanism may hold for the reduction of CrCl 2+ but not for CrF z+, which is reduced at potentials far more negative than the electrocapillary maximum.…”

Rearrangement in Coordination Complexes

“…Such a path seems feasible in view of the extraordi- nary rapidity of intramolecular cyclization reactions. This work is pertinent to the observation that NCS" ion is a much poorer bridging ligand than N 3~ in many electrontransfer reactions (234,235), a property that may arise from the possibility that NCS" transfer yields an unstable isomer whereas N 3 " with equivalent ends does not (234). The problem here is how to make the stereospecifically labeled complex at modest cost.…”