Electron transfer reactions homonuclear

Fig, 13.9 (a) Potential energy diagram for a homonuclear electron transfer reaction such as... 

The photoinduced cleavage of metal-metal bonds is now a general reaction that finds many synthetic applications e.g., many heteronuclear metal-metal bonded complexes are most conveniently prepared by irradiating solutions containing a mixture of two homonuclear compounds. A discussion of such reactions is given in 13.3, organized according to the type of metal involved. This chapter then closes with discussions of photoinduced electron-transfer reactions and pulse-radiolysis techniques. 

Rate constant data for several homonuclear electron transfer reactions involving transition metal complex ions in water are summarized in table 7.1. The striking feature of the results is that the rate constants vary over a very wide range from a low 2x 10 s to a high of 4 x lO M s. Since these... 

Table 7.1 Kinetic Data for Homonuclear Electron Transfer Reactions Involving Transition Metal Complex Ions in Water at 25°C [5] ...

The model for the inner sphere reorganization was originally based on simple electron transfer reactions involving octahedral inorganic complexes. For a homonuclear electron exchange reaction such as... 

As befits current interest, the largest number of reviews centre on metal carbonyl cluster compounds. General topics covered included electron transfer reactions, ligand and cluster transformations, and the chemistry of metal clusters containing nitrosyl and nitrido ligands.More specific topics reviewed include sulphi do-osmium carbonyl cluster compounds,and homonuclear platinum clusters.The preparations of [Nb(C0)6] [M2(CO)io(m-H)]" (M = Cr or W), Mn2(C0)aX2 and Re C0)sX (X = Cl, Br or I) are described in Volume 23 of Inorganic Syntheses. ... 

The above example illustrates the estimation of for a homonuclear reaction involving octahedral transition metal complexes. Most systems are more complicated. Flowever, if the details of the changes in bond lengths and bond angles which accompany electron transfer are known, estimates of can be made. Quantum-mechanical calculations are often required to obtain the necessary information. 

AG is the activation energy required to transfer an electron from the oxidized form of reactant A to its reduced form the same is valid for AG and reactant B. These reactions are called homonuclear electron transfer or electron exchange reactions here AG and the differences in the Coulomb interactions between reactants and products are zero. The physical interpretation of equ.IV arises from the fact that AG is the reorganisation energy required to reach the transition state (A(ox) A(red)), AG is the... 

The numbers in circles below each element are the oxidation numbers, and the numbers in the boxes are each atom s contribution to o erall charge. In both of the reactions shown, the reactants are all homonuclear diatomic molecules. Thus, we would expect no transfer of electrons from one atom to the other and the oxidation number of each is zero. For the product molecules, however, for the sake of determining oxidation numbers, we assume that complete electron transfer has taken place and that each atom has either gained or lost one or more electrons. The oxidation numbers reflect the number of electrons assumed to have been transferred. 

The predicted course of reaction between a heteronuclear pair of atoms is shown in Figure 7.2. Promotion is once more modeled with isotropic compression of both types of atom. The more electropositive atom (at the lower quantum potential) reaches its valence state first and valence density starts to migrate from the parent core and transfers to an atom of the second kind, still below its valence state. The partially charged atom is more readily compressible to its promotion state, as shown by the dotted line. When this modified atom of the second kind reaches its valence state two-way delocalization occurs and an electron-pair bond is established as before. It is notable how the effective activation barrier is lowered with respect to both homonuclear (2Vq)i barriers to reaction. The effective reaction profile is the sum of the two promotion curves of atoms 1 and 2, with charge transfer. 

In this case, one electron is transferred from Fe(II) to Cr(III), and no bonds are broken or formed. The reaction is also classified as heteronuclear, since the reactants belong to different redox couples. This means that the overall reaction involves a net change in the standard Gibbs energy, A G°. There is considerable interest also in homonuclear reactions such as... 

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