Electrochemical oxidation of basic aqueous solutions of rhodium(III)

Pergamon

0277~5387(93)EOO69-C

Polyhedron Vol. 13, No. 9, pp. 1351-1354, 1994 Copyright 0 1994 Elsevier Science Ltd Printed in Great Britain. AU rights reserved 0277-5387/94 $7.00+0.00

\

ELECTROCHEMICAL OXIDATION OF BASIC AQUEOUS SOLUTIONS OF RHODIUM(II1) IAN J. ELLISON

and R. D. GILLARD*

School of Chemistry and Applied Chemistry, University of Wales, P.O. Box 9 12, Cardiff CFl 3TB, Wales, U.K. and MONIKA MOSZNER,

MICHAL

WILGOCKI

and JOZEF

J. ZIOLKOWSKI

Institute of Chemistry, University of Wroclaw, 14F. Joliot-Curie Street, 50-383 Wroclaw. Poland (Received

16 November

1993 ; accepted 25 November

1993)

Abstract-The

electrochemical oxidation of yellow basic solutions containing hydrolysed Rh3+ results in the formation of blue superoxo-rhodium(II1). The products of oxidation (electrochemical and chemical) of aqueous rhodium(II1) are rationalized.

Electrogeneration of oxidized rhodium in oxidation states four, five and six in aqueous solutions containing rhodium(II1) species has been claimedlm4 in various studies. These states would have electronic configurations 4d5, 4d4, and 4d3, respectively. The most recent investigation, by Kiseleva, Ezerskaya and co-workers,‘,2 considered generation of rhodium(IV). Their electro-oxidation’ was typical of of of a basic solution many : oxidation [RhCl,(H,O)]‘at a controlled potential (0.380.45 V). During the initial stages of that electrolysis,’ an orange species, attributed to rhodium(IV) in [Rh(OH),12-, formed. The orange solution turned green with time, with or without further applied potential. At the stage where it became green the electrolysis was stopped, because the authors suggested’ that the green colour resulted from a Rh(IV)-Rh(II1) polymer, a species by way of which the rhodium(IV) decomposed. Our experiments extend these observations and permit us to reassess the products of such’s2 anodic oxidations : they actually involve superoxides of rhodium(II1).

*Author to whom correspondence should be addressed.

EXPERIMENTAL

Electronic spectra were recorded by means of a Hewlett-Packard 8452A spectrophotometer and ESR spectra were measured (except as indicated below) using a Jeol JS3X spectrometer at 77K ; calibration of the magnetic field was achieved using manganese(I1) markers. Voltammetric studies were performed on an EG&G Princeton Applied Research Potentiostat/Galvanostat Model 273A and an IBM PS/2 computer. The micro-voltammetric (0.5 cm3) cell (home made) was a closed vessel which was provided with platinum working electrode, Luggin capillary, salt bridge, reference (Hg/Hg,Cl,, satd. NaCl) electrode, Pt spiral auxiliary electrode, and a side arm for argon gas inlet and outlet. The bulk electrolysis measurements were performed with a Radelkis OH 404A potentiostat associated with an OH 404C digital integrator unit. The micro-coulometric (0.5 cm’) cell was a home made vessel in which the cathodic and anodic compartments were separated by a fine porosity frit. The working and the auxiliary electrode were the platinum spirals and the reference electrode was SCE with a fine salt bridge. The anodic and cathodic compartments of the cell were provided with a glass tube for argon inlet and outlet and a side arm for transferring

1351