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DISCUSStONDr. N. S. Hush (Bristol University) said: The activation energies of " outersphere " electron-exchange processes should depend strongly on the energy of re-orientation of the ligands in the first co-ordination shells of the ions to the transition-state configuration, In the thermoneutral M3+/M2+ isotopic exchange reactions of transition-group ions in aqueous solution, this re-orientation energy should vary sharply with atomic number, as a result of the crystal-field stabilization of the aquo comp1exes.S In the table below, some results are given of calculations of this term for the Fe3+/Fe2+ and V3+/V2+ exchanges, assuming these to be adiabatic outer-sphere reactions. The calculated terms for the corresponding electrode processes, i.e., M3++ e (electrode)+M2+ are also shown. These results have been obtained by a calculation similar to, but less approximate than, that described in ref.(7).The V3+/V2+ exchange, both in solution and at an electrode, is predicted to have a smaller activation energy than the Fe3+/Fe2+ exchange, because the reorientation energy of the first shell of water molecules around the ions should be smaller for the vanadium reactions. This is connected with the fact that the large crystal-field stabilization energy of the V(H20)2+ ion is accompanied by a contraction of the V . . . 0 bond distance to a value which will be very close to that in the V(H20)$+ ion, so that an unusually small expansion of the (H2O)6 shell is needed to accommodate the transferring electron. The expansion is much larger for the Fe(H20)%+ ion-probably about 0.16w. In agreement with this, the activation energy for the V3+/V2+ exchange at an electrode is lower than that for the Fe3+/Fe2+ exchange. For the homogeneous reaction, the activation energy for the Fe3+/Fe2+ exchange is reasonably well predicted. The calculated value for the V3+/V2+ homogeneous exchange, however, is much lower than the value suggested by Krishnamurty and Wahl4 on the basis of an analysis of the overall V1I1/VI1 exchange rate in 1 M HC104 solution. This high activation energy is difficult to understand, and it would be interesting to see whether it can be confirmed by an independent measurement. calculated inner-shell re-total activation expt. activation orientation energy energy energy ref. system elec-Fe3+/Fe2+ 2.5

The oxidations of the ions I-, Br-, CI-. SCN-, TI+, and IrCI,S-by bismuth(v) in perchloric acid solution all obey the rate expression -d[BiV]/dt = k[BiV]. where k = 161.4 f 8.0 s-l at [H+] = 0 . 5 ~. / = 2 . 0 ~. and t = 25 "C. Rates of reaction are practically independent of [H+] in the range 0.5-2.0~. Measurements at four different temperatures give €, = 7.1 f 0.3 kJ mol-l and A S = -187 f 1 J K -I mol-l. Mechanisms consistent with this data are discussed and that involving (i) as the rate-determining step is preferred. Poientiometric titrations I3i/OH+ Bi ' ( 0 2 -+2H+ \OH \O give EO = 2.03 rt 0.1 7 V for the half-cell reaction BiV + 2e -+ a t [H+] = 0 . 5 ~. 1 = 2 . 0 ~. and t = 25 "C.ALL elements of Group V exhibit the oxidation states 111 and V, the relative stabilities of which differ widely from element to element, as measured by electrode potential The general oxidation-reduction reaction (1) (where X, and Xrepresent a halogen molecule MI'I +x, @ h P + 2x-1 ' Stability Constants of ' 2nd edn.,

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BN1 SQ J ) 1448S u m m a r y The fact that the rates of oxidation of I-, Br-, and C1-by BiV are identical and are zero-order with respect to halide ion concentration suggests that the ratedetermining step is the production of the reactive intermediates BiIv and the hydroxy-radical.