Inhibition inner-sphere

The self-assembled monolayer had no influence on the rate of outer-sphere processes, but strongly inhibited inner-sphere processes [109]. Arias etal. [110] have also given an electrochemical characterization of the mixed self-assembled monolayer comprising chemisorbed 6TG and physisorbed guanine molecules at a HMDE. The mixed monolayer was investigated, applying chronoamperometry, cyclic voltammetry, and phase-sensitive ac voltammetry. Compact packing of the molecules within the film was found. 

The most direct evidence for surface precursor complex formation prior to electron transfer comes from a study of photoreduc-tive dissolution of iron oxide particles by citrate (37). Citrate adsorbs to iron oxide surface sites under dark conditions, but reduces surface sites at an appreciable rate only under illumination. Thus, citrate surface coverage can be measured in the dark, then correlated with rates of reductive dissolution under illumination. Results show that initial dissolution rates are directly related to the amount of surface bound citrate (37). Adsorption of calcium and phosphate has been found to inhibit reductive dissolution of manganese oxide by hydroquinone (33). The most likely explanation is that adsorbed calcium or phosphate molecules block inner-sphere complex formation between metal oxide surface sites and hydroquinone. 

PAsl00ph45 refers to 100pM of Zn presorbed prior to the 100 J,M As(III) addition at pH 4.5, and SAslOOphb refers to the simultaneous 100 J,M Zu/lOOpM As(III) addition at pH 6.0. Even though adsorbed Zn was present in the system, As(III) readily oxidized over time. However, Power et al. (2005) suggest that Zn is likely to form inner-sphere complexes on bimessite surfaces and chemisorbed Zn ions inhibit electron-transfer reactions. When Zn was present, As(in) oxidation was further suppressed by nonadsorbed and preadsorbed Zn, compared to the control system, but the preadsorbed system was more effective in interfering with electron-transfer reactions. 

Pyridine derivatives of the type known to catalyze the outer-sphere reduction of Co(III) catalyze the reduction of [Coensby The catalysis is inhibited by U +, and the catalyst is slowly consumed (102). The catalyst, for example, isonicotinamide, is reversibly reduced by an inner-sphere reaction with the In another group of papers, it is shown that can bring about reduction by an inner-sphere mechanism involving attachment that is remote from the cobalt atom. The oxidants were dinuclear complexes of the type of 6. 

This reaction is totally inhibited by ligands such as CH3CN or PPh3, a strong indication for the coordination of the reactants to cobalt, thus for an inner-sphere mechanism. When the reaction is carried out at 150°C, an insoluble black tar is formed. 

The latter two reactions proceed via the inner-sphere mechanism (see below), that is, they require access of the substrate to the central Cu(I) ion. The disproportionation reaction requires the contact of the central copper ion with a smface, preferably a Cu°(s) surface, as the formation of a Cu° atom is extremely endothermic due to the lattice energy of copper, - 301.4 kJmol (5). Thus ligands that block sterically the approach of a substrate or of a surface to the central copper ion stabilize it (19). An extreme example is 1,4,5,7.7,8,11,12,14,14-decamethyl-l,4,8,ll-tetraazacyclotetradecane, (27). Thus [Cu(I)L ] is stable even in aerated aqueous solutions (27). In analogy, some enzymes with Cud) as the active site, for example, CuSOD, inhibit disproportionation or the reaction with O2 by inhibiting the approach of two Cu(I) central ions to each other which is required for these reactions which are thermodynamically exothermic. 

Mechanisms for the electrochemical processes at mercury electrodes in solutions of [Ni(cyclam)] + and CO2 have been proposed (see Scheme 5.1 ). Scheme 5.1 shows the formation of a carbon-bonded Ni(II) complex by reaction of CO2 with Ni(cyclam)+. The formation of such a complex is considered to be a fundamental step in the mechanism of the [Ni(cyclam)] +-catalyzed electrochemical reaction. The overall process for the transformation of CO2 into CO also involves inner-sphere reorganization. Scheme 5.1 includes the formation of sparingly soluble complex containing Ni(0), cyclam and CO which is a product of the reduction of [Ni(cyclam)] + under CO. Depositation of a precipitate of the Ni(0) complex on the mercury electrodes inhibits catalysis and removes the catalyst from the cycle. The potential at which the [Ni L-C02H] + intermediate (see lower left hand of Scheme 5.1) accepts electrons from the electrode. This potential is not affected by substitution on the cyclam ring, as shown by comparison of [Ni(cyclam)] + and [Ni(TMC)] " (TMC = tefra-iV-methylcyclam)... 

The redox step is considered to proceed via two one-electron transfers in which the first is an inner-sphere reaction leading to a TF -peroxyl complex. The inhibiting role of S04 or Cl is through reduction of the number of co-ordinating sites on the metal ion. No consideration is given by the authors to an outer-sphere process. The results differ from those of a previous study and comment is made on what are seen to be deficiencies in the former investigation. 

Oxidation of cobalt(ii) bovine CA by HaOa is inhibited by specific inhibitors of CA, suggesting an inner-sphere mechanism. Reduction of the resultant cobalt-(m)-substituted enzyme by dithionite also proceeds by an inner-sphere process and the rates of binding of CN and Na to the cobalt(iii) enzyme have been determined as 6.0 x 10 and 7.0 x 10 S respectively, faster than those of... 

No evidence for a dinuclear complex of the type [TiOHUOa] was forthcoming. This again raises the question of whether identification of a hydroxy-form as the reactive species, in this case TiOH +, indicates an inner-sphere mechanism with hydroxybridging or whether loss of protons occurs, to effect greater resemblance of activated complex to final product (TiO +), in which case the mechanism may well be outer-sphere. The reaction was also studied in the presence of Inhibition occurs and... 

The reagent T1(S04)2 is the main oxidant in the reaction with iron(II) in the presence of sulfate ions. Iron(III) inhibits the reaction suggesting participation by Tl(II). An inner-sphere halide bridged mechanism is proposed for the Fe reduction of rrans-[Co(DH)2pyX] where DH is dimethylglyoximate anion and X is the halogen ion. Rate constants for both uncatalyzed and base-catalyzed pathways show an increase in the order... 

Oxidations of the molybdenum(IV) trimer M03O4 by [IrCle] and [Fe(phen)3] are inhibited by H indicating initial proton dissociation from the molybdenum complex. No mixed-valence species are detected and the product with [Fe(phen)3] is Mo(VI), while with [IrCle] , Mo(V) is produced. The difference is accounted for in terms of an inner-sphere mechanism for the latter reagent. Activation parameters are similar to those of substitution of SCN or M03O4 . 

In acetonitrile solution, two processes are also noted in the rapid reactions of alkyl radicals with [M(bipy)3] , where M is Fe, Ru, or Os. An outer-sphere oxidation of the radical forming a cation is in agreement with Marcus correlations. The cation can rearrange and subsequently decompose to form an alkene or react with solvent to give an alkyl acetamide. In the other pathway, there is considerable steric inhibition and an inner-sphere mechanism in which the radical substitutes on the phenanthroline ring is proposed. 

Inhibition by pyrophosphate ion suggests an inner-sphere mechanism in the [Mn(H2Pa07)3] oxidation of isopropyl mandelate (L) ... 

M" s for tripeptide complexes to 1.2 x 10 M s" for tetrapeptides. The former value is consistent with NMR line-broadening experiments where an upper limit of 800 M s is determined for the rate. Addition of halide or pseudohalide anions enhances the rate by axial binding to the nickel(III) complexes and allowing a bridged, inner-sphere pathway to operate. Ligands such as pyridine which cannot bridge inhibit the reaction. 

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