Labile systems redox reactions

In the case of other systems in which one or both of the reactants is labile, no such generalization can be made. The rates of these reactions are uninformative, and rate constants for outer-sphere reactions range from 10 to 10 sec b No information about mechanism is directly obtained from the rate constant or the rate equation. If the reaction involves two inert centers, and there is no evidence for the transfer of ligands in the redox reaction, it is probably an outer-sphere process. 

The first step in the reaction of ran,.s-[Fe(salpn)(H20)2l+, salpn=A(A7v-propylene-l,2-bis-salicylidiniminate, with sulfur(IV) is the formation of [Fe(S03)(salpn)(H20)], with the pH-rate profile showing greater trans-labilization by hydroxide than by water, in that traras-[Fe(salpn) (H20)2]+, reacts 10 times less rapidly than traras-[Fe(salpn)(OH)(H20)]. A limiting dissociative (D) mechanism is proposed for reaction of the latter formation of the sulfito complex is followed by a slow intermolecu-lar redox reaction (346). A similar situation prevails for the analogous irans-[Fe(salen)(H20)2]+/sulfur(IV) system (347). 

Redox substitution reactions can be photoinitiated. Taube first proposed that the photo-catalyzed substitution of PtCll- occurs by an electron-transfer process (equation 560) to give a kinetically labile platinum(III) intermediate.2040 Further work on this system has shown that the exchange occurs with quantum yields up to 1000,2041-2043 and the intermediate has beer assigned a lifetime in the fis range.2044 Recently the binuclear platinum(III) complexes Pt2(P2OsH2)4Xr (X = Cl, Br, I) have been found to show similar behavior and both photoreduction and complementary redox reactions are again proposed to explain the substitution behavior.1500... 

Transition metals, e.g. iron and copper, with their labile d-electrons system, are well suited to catalyze redox reaction. 

The reducibility of the catalyst systems was further examined using temperature programmed reduction with a 3X hydrogen/argon gas mixture. The TPR curves shown in Figure 7 illustrate the NnP oxide catalyst is not readily reduced at reaction temperatures. In contrast, the FeNo oxide catalyst begins to reduce at 250 0, and the rate of reduction is fast at temperatures of methanol ammoxidation activity (425 -475°C). The poor lability of lattice oxygen for the HnP oxide catalyst provides additional evidence for a non-redox process. 

Consequently, reduction of cobalt(III) ammines in basic solution is not favorable. A variety of reducing agents has been used to effect reaction (11). The fortunate coincidences that cobalt(III) complexes are substitution inert while cobalt(II) systems are labile and that cobalt(II) is resistant to oxidation or further reduction in acid solution offer many advantages in the study of redox processes. Not surprisingly, work with cobalt(III) complexes forms the basis for much of the present understanding of oxidation-reduction reactions. 

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