THE KINETICS AND MECHANISM OF CONCURRENT REACTIONS OF ACETYLENE HYDROGENATION AND HYDROIODINATION IN THE RH(III)-H-I--H2O SYSTEM()

The kinetics and mechanism of the new catalytic reaction of hydroiodin ation and of the reduction of acetylene to ethylene in the Rh(III)-NaI -H+-H2O system at 80-degrees-C are studied. The results obtained allow us to suggest that the rate-determining step in acetylene conversion is the formation of a sigma-vinyl complex of Rh(III) as a result of th e reactions of reversible formation of acetylide, Rh(III)-C=CH reducti on by iodide ions to Rh(I) acetylides, and protolysis of the latter, r esulting in the formation of a pi-acetylenic complex of Rh(I). Acetyle ne is recovered in the course of the decomposition of the pi-acetyleni c complex, and its protonation leads to the formation of the sigma-vin yl complex of Rh(III). Reductive elimination accompanying the Rh(III)- CH=CH2 decomposition under the action of I- produces vinyl iodide. Eth ylene evolves at the stage of Rh(III)-CH=CH2 reversible reduction by i odide ions to the corresponding derivatives of Rh(I) and at protolysis of the latter.