Journal of The Chemical Society, Chemical Communications NUMBER 1111975 4 JUNE Formation of CrO:+ in the Oxidation of Chromium(I1)by Molecular Oxygen By ROBINM. SELLERS*? and MICHAELG.SIMIC (Hahn-Meitner-Institut fiir Kernforschung Berlin GmbH, Bereich Strahlenchemie, 1 Berlin 39, West Germany) Summary Cr2+ reacts with molecular oxygen with a rate constant of (1.6 f0.2) x 1081mol-1s-1 to give a com- plex, Cr0,2+, with A,,, 245 and 290nm, E = 7800 and 3200 1 mol-l cm-l, and which is long lived when [Cr2+] <[O,l* isCHROMIUM(II) rapidly oxidised by molecular oxygen according to the stoicheiometry in equation (1) .1 The 4Cr2++ 0, + 4H+ = 4Cr3+ + 2H,O (1) final product of the reaction is known' to be [Cr(OH),CrI4+, although the mechanism of its formation remains un-certain.lS2 We have studied the reaction by pulse radiolysis.Pulse radiolysis of aerated aqueous solutions containing lo-, M Cr3+ + 0.5~ButOH produced the spectra shown in the Figure. Following the pulse there was a first order build-up, and then a much slower decay to a plateau. Under these conditions the hydrated electrons produced in the radiolysis of water, reaction (2),(concentration ca. 2 x M) rapidly reduce Cr3+ to Cr2+, reaction (3),which in turn reacts with O,, reaction (4). ButOH was added to scavenge the hydroxyl radicals and convert them into unreactive ButO-O. radicals. The pseudo-first-order build- up of absorption was linearly dependent on [O,] (2.6-13 x M), but independent of [Cr3+] (2-10 x M) and pH (2.6-4.3), and is attributed, therefore, to the formation of the product of Cr2+ and 0,.A value of k, = (1.6 f0.2) x 108 1mol-1 s-l was calculated from the pseudo-first order rate constants measured. As a check on this k, was also esti- mated by a competition method using benzoquinone. This H,O -+ e-.,, OH, H, H,, H,O,,H+ (2) e,+ Cr3+ 3 Cr2+ (3) Cr2+ + 0, +-CrO,*+ (4) gave k, = (1.9 f0.3) x loe1 mol-1 S-1 taking k[Crg+ + benzoquinonel = (3.2 f0.3) X 10%1 m0l-l S-'.' The Par-I Wavelength /nm FIGURE.Spectra of transients produced by the pulse radiolysis of aerated solutions of M Cr*+ + 0-5 M ButOH, pH = 3.4, dose = ta.600 rad/pulse. (0)measured 60 ps after the pulse;(0)4-6 ms after the pulse (spectrum of CrOSa+); (A)difference spectrum, and (----) spectrum of ButO.0.radical taking G = 2.8, from ref. 4. t Present address :Research Department, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucester- shire GL13 9PB. An1 tial decay of absorption noted at longer times is probably due to the disappearance of BuQ-0. radicals. In support of this it was found that the difference of the spectra at 50 ps and of the plateau at 4-5 ms was of the same shape and magnitude (taking G = 2.8) as an authentic ButO.O. radical ~pectrum.~ The product of the reaction Cr2+ and 0, has, therefore, hmax245 and 290 nm, E 7800 and 3200 1 mol-l cm-l, respectively, assuming it is formed with a yield of G = 2.8. The spectrum measured at 4.5 ms is assigned to the species Cr0,2+.The reaction Cr2+ and 0, cannot be an electron transfer resulting in 0,-, or its protonated form, HO, [pK,(H02) = 4.88*] because (i) the spectrum is sub-stantially different from that of either 0,-or H0,,6 and (ii) the observation of a competition between 0, and benzo- quinone for Cr2+ implies that the product of reaction (4) is unreactive with benzoquinone, while 0,-transfers an electron to benzoquinone with K = 9-8 K 108 1 mol-1s-1.6 J.C.S. CHEM.COMM.,1975 Using the a.c. conductivity technique no post-pulse change in conductivity was found at pH 2.6-4.3 and hence reaction (4) is not accompanied by loss or uptake of protons. No decay of CrO,a+ was detected. Indeed a permanent drop in the photomultiplier current was observed in these solutions at wavelengths near 260 nm, where Cr022+ absorbs intensely, and it seems that the species is quite long lived under the conditions of our experiments.Earlier workers1 t2 have found no evidence for a long-lived intermediate such as CrOS2+ probably because under their conditions ( [Cr2+]> [O,]) reaction (5) occurs. Formation of the ultimate product, [Cr (OH) ,Cr] *+, then presumably takes place Cr029++ Cr2+-+ Cr02Cr4+ (5) through further reduction of the peroxy intermediate, Cr02Cr4+, by 2 equiv. 'of Cr2+ followed by protonation. (Received, 18th February 1975; Corn. 201.) R. W. Kolaczkowski and R. A. Plane, Inorg. Chem., 1904.3, 322. M. Ardon and G. Stein, J. Chem. SOC.,1956, 2095; M. Ardon and R. A. Plane, J. Amer. Chem. SOC.,1959, 81, 3197; J. A. Laswick and R. A. Plane, ibid., p. 3564. R. M. Sellers and M. Simic, to be published. 'M. Simic, P. Neta, and E. Hayon, J. Phys, Chem.. 1969, 73, 3794. D. Behar, G. Czapski, J. Rabani, L. M. Dorfman, and H. A. Schwarz, J. Phys. Chem., 1970, 74, 3209. a R. L. Willson, Trans. Faraday SOC.,1971, 67, 3020; M. Simic and E. Hayon, Biochem. Biophys. Res. Comm.. 1973, 50, 364.