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The kinetics of the reaction of vanadiurn(l1) with the binuclear complex (NH3)5Co-NH2*Co(NH3)55+ have been studied in aqueous perchloric acid solutions of ionic strength p = 2.0 and 0.4M a t temperatures from 10.4 to 31.2". The reaction is independent of the hydrogen-ion concentration (indicating an outer-sphere reaction), and is at least predominantly by a mechanism involving successive one-equivalent reactions :At p = 0.4M activation parameters for (i) are AH1 = 9.5 ( 2 ~0 . 8 ) kcal. mole-l and A S = -31 (*3) e.u., and at 25" reaction (i) is some thirty times faster than (ii). The activation enthalpies for the two reactions are essentially the same and the difference in rates is due to the difference in activation entropies.< F-, the relative rates being similar (they are 2-4 times greater) to those for reaction (ii). The nature of the activated complex is discussed.Reaction (i) is catalysed by anions CI-< TAUBE and his colleagues1t2 studied the reaction of vanadium(r1) with CO(NH3)63' and the catalytic effect which anions C1-, SO,2-, and Fhave on the reaction.We give here details of a parallel study with the binuclear complex (NH,) ,COON H,*Co (N H,) , , +. The hexaniminecobalt (111) complex is extremely inert to substitution and since the rate of the reaction of V2+ with Co ( NH3)63+ is independent of hydrogen-ion concentration (thus excluding bridging by an amide group), the reaction is believed to proceed by an outer-sphere mechanism. The decammine-pamidodicobalt (111) complex is also inert to substitution and the reaction with \Tanadiuni(Ir) is likewise expected to be outer-sphere. An important difference, however, is that the binuclear complex is potentially a two-equivalent oxidising agent, and since vanadium(I1) can be oxidised to vanadium(Iv), a two-equivalent reaction is clearly possible. Baker (NH,),CO*NH,-CO(NH,)~~+ with vanadium(I1) is significantly faster than the corresponding reaction of Co(NH3),3+ with vanadium(I1). Possible reaction sequences are therefore a fast step (2) followed by the slower reaction (3) or alternatively a two-equivalent Hi-V2+ + (NHJ5Co.NH2*Co(NHJj5+ -V 3 + + Co2f + CO(NH,),~+ +-5NH4+ (2)

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