The p-amido-y-superoxo-complex (en),Co.p( NH,,O,) ~Co(en),~+ and the p-amido-p-peroxo-complex (en),Co.p( NH,.O,) .C~(en),~+ both react with sulphite ions to give the p-amido-p-sulphato-complex (en),Co*p(NH,,SO,)*-C ~( e n ) , ~+ . The p-amido-p-superoxo-reaction proceeds with the intermediate formation of the p-amido-p-peroxocomplex and the sulphite radical. Stoicheiometries a t pH 1-3 are (complex :sulphite) 2 : 3 for the superoxo-and 1 : 1 for the peroxo-complex, and rate laws are of the form k,[superoxo] [SO3,-] and k,[peroxo] [H,S03] respectively, where k, = 3.80(&0.07) x l o 4 I mol-l s-1 and kD = 55.4(*6.1) I mol-l s-l, both at 1 -4 "C and / = 1 .OM ( LiC104). The protonated p-peroxo-complex may be the reactive species in the latter, in which case the rate law can be expressed in an alternative form. The complexes also react with nitrite ions to give the p-amido-p-nitritocomplex (en),C0~p(NH,,N0,)*Co(en),~~. With arsenite ions a different reaction path is operative, and the product is the p-amido-p-hydroxo-complex, (en),Co.( NH,,OH) ~Co(en),~+.. Chemistry, The University, Leeds LS2 9JT REACTIONS of molecular oxygen bonded to metals are of the p-amido-p-peroxo-and p-amido-p-superoxoimportant processes in the chemistry of living organisms dicobalt (1x1) complexes with sulphite, nitrite, and and any insight into the mechanism of the stepwise arsenite ions respectively, and consider different mechanreduction of oxygen (02+ O,-+ OZ2-, etc.) is isms which are apparent. One unprotonated and highly relevant. We report in this study the reactions two singly protonated forms of the p-peroxo-complex [4/009 Received, 3vd Januavy, 19741 l9 A. I. Vogel, ' Quantitative Inorganic Analysis,' Longmans, London. 1960.