Inner ligand bridged

Fig. 6.1b) in which twelve inner ligands bridge the edges of the Me octahedron, and six outer ligands occupy apical positions, predominate. These units are found in reduced zirconium, niobium, tantalum, and rare-earth halides, and niobium, tantalum, molybdenum and tungsten oxides [la, 6, 10]. 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

Figure 1. Idealized structures of an octahedral ML8 complex (a), a face-capped octahedral M6X8L6 cluster (b), and an edge-bridged octahedral M6X12L6 cluster (c). Black, shaded, and white spheres represent metal atoms M, inner-ligands X, and terminal-ligands L, respectively. Each structure conforms to Oh symmetry.

If ligands are involved in the formation of discrete intermediates or if metal ions become ligand-bridged, the process is designated as inner-sphere (IS) electron transfer [52]. In these cases, the electronic interaction between the redox centers is increased substantially, and leads to a lowering of the activation barrier (and hence to increased rates) for the ET reaction [13, 15, 53],... 

Workers in the Soviet Union have made some copper complexes of the mono- (31) and dinuclear (32) systems (79MI3 80MI2). X-Ray photoelectron spectra, which give information on inner-electron bridging energies of the ligand W, P, and O and of the Cu and Cl, in this case provided no useful evidence on the nature or position of the bonding. 

Experimental estimates of <5r/c , are relatively difficult to obtain. While they can, in principle, be extracted from temperature-dependence studies, this approach is complicated by uncertainties in the entropic term (Sect. 4.3). An alternative method has recently been described for some Cr(III) reductions which involves comparing the work-corrected rate constants, kco , with unimolecular rate constants, ket, for structurally related reactants that reduce via ligand-bridged pathways [30]. Provided that the corresponding outer- and inner-sphere pathways involve the same activation barrier (Sect. 4.6) and the latter also follow adiabatic pathways, we can write [30]... 

The aqua ion has been extensively used as a reductant in studies on the mechanism of electron transfer reactions, best exemplified by the classical example of the reaction of Cr2+(aq) with [Com(NH3)5X]2+. This reaction proceeds via an inner-sphere (ligand-bridged) mechanism, as in the general reaction sequence... 

Other reactions that follow an inner-sphere mechanism have been studied to determine which ligands bridge best. The overall rate of reaction usually depends on the first two steps (substitution and transfer of electron), and in some cases it is possible to draw conclusions about the rates of the individual steps. Eor example, ligands that are reducible provide better pathways, and their complexes are more quickly reduced. Benzoic acid is difficult to reduce, but 4-carboxy-N-methylpyridine is relatively easy to... 

Fto. 10. Intercluster bridging between M6Y8, + clusters. The vertices of the cube represent the inner ligands Y. 

Figure 3.6.1 Outer-sphere and inner-sphere reactions. The inner sphere homogeneous reaction produces, with loss of H2O, a ligand-bridged complex (shown above), which decomposes to CrCl(H20) + and Co(NH3)5(H20). In the heterogeneous reactions, the diagram shows a metal ion (M) surrounded by ligands. In the inner sphere reaction, a ligand that adsorbs on the electrode and bridges to the metal is indicated in a darker color. An example of the latter is the oxidation of Cr(H20)5 at a mercury electrode in the presence of Cl or Br .

In principle, any of the steps in Scheme 2 can be rate limiting from the diffusion controlled formation of the association complex to the substitutional breakup of the successor complex. Because of the multiplicity of steps, a detailed interpretation of an experimentally observed rate constant can be extremely difficult. However, in some cases it has been possible to obtain direct or indirect information about the electron transfer step in an inner-sphere reaction using chemically prepared, ligand-bridged dimeric complexes. For example, reduction of the Co -Ru precursor to the product shown in equation (7) by Ru(NH3)6 + or Eu + occurs selectively at the Ru " site. The initial reduction to give Ru is followed by intramolecular electron transfer from Ru to Co which is irreversible since the Co site is rapidly lost by aquation. ... 

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