Outer-sphere complex Substitution reactions

Although somewhat more stable than its hexaammine relative, the air-sensitive [Co(en)3]2+ is still substitutionally labile and racemizes rapidly in solution. Chiral discrimination in its (racemic) solutions has been observed in outer sphere electron transfer reactions with optically active oxidants including [Coin(EDTA)], 209,210 [Cr(ox)3]3-,211,212 Co111 oxalate, malonate, and acetylacetonate (acac) complexes.213... 

DR. DAVID RORABACHER (Wayne State University) A point which is frequently overlooked is that the calculations generally applied for determining the extent of ion-pair (or outer-sphere complex) formation in substitution reactions may be overly simplistic. There are many types of interactions which tend to perturb the extent of outer-sphere complex formation relative to the purely statistical calculation commonly made which takes into account only the reactant radii and electrostatic factors. 

The aquated Co(III) ion is a powerful oxidant. The value of E = 1.88 V (p = 0) is independent of Co(III) concentration over a wide range suggesting little dimer formation. It is stable for some hours in solution especially in the presence of Co(II) ions. This permits examination of its reactions. The CoOH " species is believed to be much more reactive than COjq Ref. 208. Both outer sphere and substitution-controlled inner sphere mechanisms are displayed. As water in the Co(H20) ion is replaced by NHj the lability of the coordinated water is reduced. The cobalt(III) complexes which have been so well characterized by Werner are thus the most widely chosen substrates for investigating substitution behavior. This includes proton exchange in coordinated ammines, and all types of substitution reactions (Chap. 4) as well as stereochemical change (Table 7.8). The CoNjX" entity has featured widely in substitution investigations. There are extensive data for anation reactions of... 

Inner sphere oxidation-reduction reactions, which cannot be faster than ligand substitution reactions, are also unlikely to occur within the excited state lifetime. On the contrary, outer-sphere electron-transfer reactions, which only involve the transfer of one electron without any bond making or bond breaking processes, can be very fast (even diffusion controlled) and can certainly occur within the excited state lifetime of many transition metal complexes. In agreement with these expectations, no example of inner-sphere excited state electron-transfer reaction has yet been reported, whereas a great number of outer-sphere excited-state electron-transfer reactions have been shown to occur, as we well see later. 

Coordinatively saturated metal complexes that are kinetically inert with respect to ligand substitution may undergo outer-sphere electron transfer reactions with dioxygen. Typical examples include oxidations of six-coordinate chromium(II) complexes29 (Equation 4.6) and oxidations of polyoxometallate anions.30... 

Outer-sphere complexes have been discussed by several authors in the context of ligand substitution reactions of metal complexes 29, 30). We consider here an example relevant to reaction (2), the outer-sphere association of TMPyP(- -4) with MA , x=0, 1, 2. 

The importance of a nonadiabatic path for outer-sphere electron transfer reactions of Eu(III)/Eu(Il) was again examined by Yee et al. (1983) via a study of a series of reactions with Eu(III)/(II) cryptates (table 12). The cryptate (polyoxadiazamacrobi-cyclic) ligands form thermodynamically stable and substitution inert complexes with both Eu(lll) and Eu(ll), markedly changing the primary coordination spheres of the Eu ions. The dramatic variation in the values for the Eu exchange reactions with such a change is demonstrated by the respective calculated values for EUavalues calculated from the cross reactions are consistent with the values of the Franck-Condon barriers estimated from structural data. 

Copper(II) and zinc(II) are two of the more labile divalent metal ions and as a consequence the former is too labile for its water exchange rate to be determined by the NMR methods which utilize the paramagnetism of other divalent first-row transition metal ions, while the latter is diamagnetic and such NMR methods cannot be applied. However, it has been shown that water exchange rates and mechanisms can be deduced with reasonable reliability from simple ligand substitution studies, and this is one of the reasons for a recent variable-pressure spec-trophotometric SF study of the substitution of 2-chloro-l,10-phenanthroline on Cu(II) and Zn(II). The observed rate constants for the complexation reaction (kc) and the decomplexation reaction (k ) and their associated activation parameters for Cu(II) and Zn(II) are kc(298 K) = 1.1 x 10 and 1.1 x 10 dm mol" s", AH = 33.6 and 37.9 kJ mol", A5 = 3 and -2JK- mol", AV = 7.1 and 5.0 cm" mol", k 29S K) = 102 and 887 s", AH = 60.6 and 57.3 kJ mol", A5 = -3 and 4 J K" mol" and A V = 5.2 and 4.1 cm" mol". These data are consistent with the operation of an mechanism for the rate-determining first bond formation by 2-chloro-l,10-phenanthroline with the subsequent chelation step being faster [Eq. (18)]. For this mechanistic sequence (in which [M(H20)6 L-L] is an outer-sphere complex) it may be shown that the relationships in Eq. (19) apply. 

The study of the effect of solvent upon the rate of complexation has revealed details of the reaction mechanism, and it is hoped that a study of the solvent-dependence of activation volumes may permit further elucidation of metal-ligand, metal-solvent and solvent-solvent interactions. According to the Eigen-Wilkins mechanism for ligand substitution reactions ( )(5)5 the first step is formation of an outer-sphere complex, characterised by an equilibrium constant Ki2 Subsequently the ligand enters the first coordination sphere and the forward rate constant for this step is identified with kg for exchange of... 

As regards intimate mechanism, electron transfer reactions of metal complexes are of two basic types. These have become known as outer-sphere and inner-sphere (see Chapter 4, Volume 2). In principle, an outer-sphere process occurs with substitution-inert reactants whose coordination shells remain intact in... 

X = H2O, OH , CP, NCS , and Nj in all these cases the rates of reduction are rapid compared with the rates of substitution of X" in Fe(H20)6. No evidence is evinced for formation of VX complexes. It is noteworthy that OH exerts little effect on the rate of this reaction although it has a pronounced influence on the rate of other reactions of Fe(IlI) . This result, together with the observation that the other anions have similar effects on the rate, would seem to indicate that the FeX " reactions proceed via an outer sphere path. The... 

The complex has been separated by ion exchange and characterised by direct analysis . The complex has a distinctive absorption spectrum (Fig. 11), quite unlike that of Np(V) and Cr(III). The rate coefficient for the first-order decomposition of the complex is 2.32 x 10 sec at 25 °C in 1.0 M HCIO. Sullivan has obtained a value for the equilibrium constant of the complex, K = [Np(V) Cr(III)]/[Np(V)][Cr(III)], of 2.62 + 0.48 at 25 °C by spectrophotometric experiments. The associated thermodynamic functions are AH = —3.3 kcal. mole" and AS = —9.0 cal.deg . mole . The rates of decay and aquation of the complex, measured at 992 m/t, were investigated in detail. The same complex is formed when Np(VI) is reduced by Cr(II), and it is concluded that the latter reaction proceeds through both inner- and outer-sphere paths. It is noteworthy that the substitution-inert Rh(lII), like Cr(III), forms a complex with Np(V) °. This bright-yellow Np(V) Rh(III) dimer has been separated by ion-exchange... 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

As indicated under section 2.2. the overall result is the same as that of an insertion reaction, the difference being that insertion gives rise to a yw-addition and nucleophilic attack to an anri-addition. Sometimes the two reaction types are called inner sphere and outer sphere attack. There is ample proof for the anti fashion the organic fragment can be freed from the complex by treatment with protic acids and the organic product can be analysed [19], Appropriately substituted alkenes will show the syn or anti fashion of the addition. The addition reaction of this type is the key-step in the Wacker-type processes catalysed by palladium. 

Only in a limited number of instances will the value of k and its associated parameters be useful in diagnosing mechanism. When the redox rate is faster than substitution within either reactant, we can be fairly certain that an outer-sphere mechanism holds. This is the case with Fe + and RuCP+ oxidation of V(II) and with rapid electron transfer between inert partners. On the other hand, when the activation parameters for substitution and redox reactions of one of the reactants are similar, an inner-sphere redox reaction, controlled by replacement, is highly likely. This appears to be the case with the oxidation by a number of Co(III) complexes of V(II), confirmed in some instanees by the appearance of the requisite V(III) complex, e.g. 

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