Precursor complex outer-sphere electron transfer

Several studies of bimetallic complexes in which the donor and acceptor are linked across aliphatic chains have demonstrated that these are generally weakly coupled systems. " Studies of complexes linked by l,2-bis(2,2 bipyridyl-4-yl)ethane (bb see Figure 5), indicate that these are good models of the precursor complexes for outer-sphere electron-transfer reactions of tris-bipyridyl complexes. A careful comparison of kinetic and spectroscopic data with computational studies has led to an estimate of //rp = 20cm for the [Fe(bb)3pe] + self-exchange electron transfer. In a related cross-reaction, the Ru/bpy MLCT excited state of [(bpy)2Ru(bb)Co(bpy)2] + is efficiently quenched by electron transfer to the cobalt center in several resolved steps, equations (57) and (58). ... 

In the absence of ion pairing and rate limitation by solvent dynamics, the volume of activation for adiabatic outer-sphere electron transfer in couples of the type j (z+i)+/z ju principle, be calculated as in equation 2 from an adaptation of Marcus-Hush theory. In equation 2, the subscripts refer respectively to volume contributions from internal (primarily M-L bond length) and solvent reorganization that are prerequisites for electron transfer, medium (Debye-Huckel) effects, the Coulombic work of bringing the reactants together, and the formation of the precursor complex. 

In contrast, similar plots of AG against AGet for the oxidants TCNE and hexa-chloroiridate(IV) deviate substantially from the simulated curve for outer-sphere electron transfer (see Figures 19B, C). Moreover, the most pronounced deviations are observed with the least hindered tetraalkyltin donors. The fact that steric effects are only observed in the latter cases, but not with the FeL3 acceptor, leads to the conclusion that the inner coordination spheres of the tetraalkyltin donors are perturbed by TCNE and hexachloroiridate in the ET transition state. In other words, the electron transfers to TCNE and iridate(IV) exhibit strong inner-sphere character and thus occur from wai i-five-coordinate precursor complexes reminiscent of a variety of trigonal-bipyramidal structures known for tin(IV) derivatives, i.e. 

This expression relates the second-order rate constant, k, for an outer-sphere electron transfer reaction to the free energy of reaction, AG°, with one adjustable parameter, X, known as the reorganization energy. Wis the electrostatic work term for the coulombic interaction of the two reactants, which can be calculated from the collision distance, the dielectric constant, and a factor describing the influence of ionic strength. If one of the reactants is uncharged, Wis zero. In exact calculations, AG should be corrected for electrostatic work. The other terms in equation 46 can be treated as constants (Eberson, 1987) the diffusion-limited reaction rate constant, k, can be taken to be 10 M" is the equilibrium constant for precursor complex formation and Z is the universal collision frequency factor (see Eberson, 1987). 

It is important to note for the following discussion that in electron-transfer processes the reductant s highest occupied molecular orbital (HOMO) should combine with the oxidant s lowest unoccupied molecular orbital (LUMO) of the same symmetry to ensure proper overlap of reductant and oxidant orbitals to initiate electron transfer. That is, electron transfer will occur readily from n to n orbitals on different species or from a to a but not n to a in a linear arrangement of atoms [e.g., A-B-C in Appendix I (following references at the end of this chapter)]. In the case of outer-sphere electron-transfer processes, n- to 7r-electron transfers are favored over a to a because (1) such transfers do not require major changes in bond lengths in the precursor complex (lower activation energy) and (2) the n orbitals are more diffuse or better exposed than a orbitals. This process is well documented for transition metals. For inner-sphere electron-transfer processes, both n- to n- and a- to n-electron transfers are most favored (Purcell and Kotz, 1980). 

Comparison with penta-amminecobalt(m) complexes with the same carboxylato-ligands confirms that the bridging carboxy-group is not available for adjacent attack. The -phthalato-complex is believed to react via the precursor complex (19), but electron transfer via the aromatic ring system is not invoked. An outer-sphere type of electron transfer is suggested, on the basis of the value of and the ratio of rates of chromium(ii) and vanadium(n) reductions. ... 

Figure 93 Three-step mechanism for outer-sphere electron-transfer reactions. Line 1. Chemical species and processes. Line 2. R are the reactants I is the precursor intermediate complex I is the successor intermediate complex P are the products j is the activated complex. Line 3. a is the association to I et is the electron-transfer step d is the dissociation of I to products b, c is the ligand and solvent reorganisation. Line 4. Free-energy changes.

The homogeneous outer sphere electron transfer reactions in solution occur at a rate that is noticeably Icj er than the diffusion rate. This peculiar behaviour has been explained through a three-step mechanism formation of a precursor complex from the separated reactants, actual electron transfer within this complex to form a successor complex and dissociation of the latter complex into separated products. The reaction rate is usually controlled by the electron transfer step, this step being governed by the Franck-Condon principle. This principle is embodied in classical electron transfer theories using an activated-complex formalism in which the electron transfer occurs at the intersection of two potential energy surfaces, one for the reactants and the other for the products. This implies that the second step necessarily involves the reorganization of the solvent before and after the electron transfer itself is produced. So, it is obvious that solvent must play an essential role in the rate of electron transfer reactions in solution. 

The Marcus therory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters1139"1425. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 10.4). 

Similarly, inner-sphere and outer-sphere mechanisms can be postulated for the reductive dissolution of metal oxide surface sites, as shown in Figure 2. Precursor complex formation, electron transfer, and breakdown of the successor complex can still be distinguished. The surface chemical reaction is unique, however, in that participating metal centers are bound within an oxide/hydroxide... 

At 25°C and p = 0.1 M, the values of are 10 -10 M and those of k are 10 -10 s depending on the identity of L. The internal electron transfer rate in an outer-sphere complex can thus be analyzedwithout considering work terms or, what is equivalent, the equilibrium controlling the formation of precursor complex. This favorable situation is even improved when the metal centers are directly bridged. The relative orientation of the two metal centers in a well-established geometry can be better treated than in the outer-sphere complex (Sec. 5.8). 

The rate-controlling step in reductive dissolution of oxides is surface chemical reaction control. The dissolution process involves a series of ligand-substitution and electron-transfer reactions. Two general mechanisms for electron transfer between metal ion complexes and organic compounds have been proposed (Stone, 1986) inner-sphere and outer-sphere. Both mechanisms involve the formation of a precursor complex, electron transfer with the complex, and subsequent breakdown of the successor complex (Stone, 1986). In the inner-sphere mechanism, the reductant... 

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