Reduction of some acidopentaaquochromium(III) complexes at the mercury electrode: effect of maximum suppressors and free ligand

The complexes [CrNCS(H2O)5]2+, [CrCl(H2O)5]2+, [Cr(H2O)5′2+ and [Cr(H2O)6]3+ were studied by polarography and cyclic voltammetry in aqueous solution. It is shown that Triton X-100 and gelatine shift the polarographic waves to more negative potentials. In 1M NaClO4---HClO4 the half-wave potentials in the absence of maximum suppressor are (in Volts vs. SCE): CrNCS2+: −0·675; CrCl2+:−0·62; CrF2+:−1·105; Cr3+:−0·90 (water omitted in the formula). All reductions are irreversible. Replacement of NaClO4 by NaSCN makes the reduction of CrNCS2+ and CrCl2+ more reversible and shifts 50 mV per log [SCN−] unit to more negative potentials for CrNCS2+ and 66 mV for CrCl2+. It is suggested that CrNCS2+ and CrCl2+ may be reduced via a mechanism analogous to the inner-sphere mechanism in homogeneous reductions.