Chloride catalysis in some reductions by stannous ion

The rates of reduction of methyl orange by stannous ion and of reduction of ferricinium ion by stannous ion were measured spectrophotometrically at 25°C in perchloric acid-hydrochloric acid solutions of ionic strength 2.0. The methyl orange reduction rate were found to agree with the rate expression, Equation (36). The rate of reduction of ferricinium Rate=11250M.O. Sn+2 Cl-3 36 ion was found to agree with the rate expressions, Equation (37). Rate=24.5Fe C5H5+ 2Sn+2 Cl-4 37 By factoring the rate expression into two bimolecular parts the slow step may be interpreted as an electron-transfer between various complex ions;The methyl orange reduction is postulated as an electron-transfer between the SnCl3- ion and the protonated azo group. The ferricinium reduction rate is explained by a postulated electron-transfer between the SnCl3- ion and a ferricinium chloride complex, Fe(C5H5)2Cl. Spectroscopic evidence was obtained confirming the presence of small equilibrium amounts of the postulated ferricinium chloride complex in solution;The presence of an additional chloride ion in the transition state of the ferricinium ion reduction suggests that the iron atom is effectively shielded by the cyclopentadienyl rings toward electron-transfer, and that the chloride bridge is a much more efficient electron-transfer path