Kinetics and mechanism of the reduction of rhodium(III) complexes Rh(H2O)5X2+ by chromium(2+)

Abstract

Reduction of the yellow rhodium(III) ions (Rh(H/sub 2/O)/sub 6/)/sup 3 +/, (Rh(H/sub 2/O)/sub 5/Cl)/sup 2 +/, and (Rh(H/sub 2/O)/sub 5/Br)/sup 2 +/ by Cr/sup 2 +/ in aqueous HClO/sub 4/ solution to give green rhodium(II) aqua dimer Rh/sub 2//sup 4 +/ has been studied by the stopped-flow method at 25 /sup 0/C, I = 20.M (LiClO/sub 4/). An (H/sup +/)/sup -1/ term is dominant in the reduction of (Rh(H/sub 2/O)/sub 6/)/sup 3 +/ (pK/sub a/ = 3.4), and rate constants (M/sup -1/ s/sup -1/) for the different oxidants are (Rh(H/sup 2/O)/sup 5/OH)/sup 2 +/ (2.3 x 10/sup 5/), (Rh(H/sup 2/O)/sup 5/Cl)/sup 2 +/ (2.2 x 10/sup 3/), (Rh(H/sup 2/O)/sup 5/Br)/sup 2 +/ (>8 x 10/sup 3/). The products (Cr(H/sup 2/O)/sup 5/Cl)/sup 2 +/ and (Cr(H/sup 2/O)/sup 5/Br)/sup 2 +/ have been identified, and reactions are of the inner-sphere type. Addition of Cr/sup 3 +/ (0.03 M) and Co/sup 2 +/ (0.03 M) as possible reactants with monomeric Rh/sup 2 +/ had no effect on the rate of formation of Rh/sub 2//sup 4 +/. Free halide ions react with Rh/sub 2//sup 4 +/ and induce Rh metal formation. Reduction of (Rh(H/sub 2/O)/sub 6/)/sup 3 +/ with V/sup 2 +/ also givesmore » Rh metal, probably as a result of outer-sphere generation of monomeric Rh/sup 2 +/. 16 references, 2 figures, 2 tables.« less