Redox Properties of Metal Complexes

There are two types of electron transfer mechanisms for transition metal species, outer- and inner-sphere electron transfers. The outer-sphere electron transfer occurs when the outer coordination sphere (or solvent) of the metal centers is involved in transferring electrons. This type of transfer does not imply reorganization of the inner coordination sphere of either reactant. An example of this reaction is given in Equation (3.54)  

Inner-sphere electron transfer involves the inner coordination sphere of the metal complexes, and normally takes place through a bridging ligand. A classic example studied and explained by Taube (1953) is given in equation 3.55  

In this reaction, the chloride that was initially bound to Co(III), the oxidant, becomes bound to Cr(III) in complexes that are kinetically inert. The bimetallic complex [Co(NH3)5(p-Cl)(Cr(H20)5] + is formed as an intermediary, wherein p-Cl indicates the chloride bridges between the Cr and Co atoms, serving as a ligand for both. The electron transfer occurs across a bridging group from Cr(II) to Co(III) to produce CraiI)andCoai). 

Redox changes occurring in a biological environment can have a pronounced influence on the overall toxicological (biological) response elicited by a metallic complex. 

A typical example is mercury, which can exist in two oxidation states and the free state. These species exhibit marked differences in uptake, distribution, and toxicological effects. The three forms are governed by the following disproportion reaction (Equation [3.56])  

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Many analytes that have basic sites prone to protonation, display pH-dependent electrochemistry. The redox properties of metal complexes of H2O, OH, and often display pH-dependent electrochemistry as demonstrated by Meyer et al. for the complex [M(tpy)(bpy)0] + (M = Ru or Os tpy = 2,2, 2"-terpyridine). These complexes have been studied probing their electrochemistry over a wide range of pH. CV and DPP were used to determine Ei/2 for the Ru and Ru redox couples of the complex [Ru(tpy)(bpy)0] + from pH 0 to 13 (Figure 4) and the Nemst equation (2) was used to fit the data. 

Spectroscopic properties of [Ru(bpy)3] " ", and the effects of varying the diimine ligands in [Ru(bpy)3 L ] + (L = diimine) on the electronic spectra and redox properties of these complexes have been reviewed. The properties of the optical emission and excitation spectra of [Ru(bpy)3] +, [Ru(bpy)2(bpy-d )] + and [Ru(bpy-d )3] " " and of related Os, Rh , and Pt and Os species have been analyzed and trends arising from changes in the metal d or MLCT character in the lowest triplet states have been discussed. A study of the interligand electron transfer and transition state dynamics in [Ru(bpy)3] " " has been carried out. The results of X-ray excited optical luminescence and XANES studies on a fine powder film of [Ru(bpy)3][C104]2 show that C and Ru localized excitation enhances the photoluminescence yield, but that of N does not. 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

Transition metal ions with organic radicals exist in the active sites of metalloproteins. The best understood example is galactose oxidase, which features a single Cu(II) ion coordinated to a modified tyrosyl radical. Many combined experimental and theoretical studies have focused on electronic properties of metal complexes with redox active ligands, yet reactivity beyond characterization has been limited. We will demonstrate the influence of the metal complex redox state on H2 activation by anilino-phenolate noninnocent ligands. 

Hybrid materials which combine the electronic conductivity of thiophene-conjugated polymers and the redox and optical properties of metal complexes are being developed to take advantage of synergistic electronic interactions. 

Mixed donor ligands present a challenge to the coordination chemist as a result of structural aspects discussed above. Further, the presence of donors of distinctly different character, such as an N-donor and an S-donor, influence the way they select metal ions, and the stabihty, spectroscopic, and redox properties of their complexes. 

Most of the interesting redox properties of metal-polypyridine complexes originate from the electron-transfer activity of polypyridine ligands themselves. The free bpy ligand and its analogs are sequentially reduced in two one-electron electro-chemically reversible steps, producing the corresponding radical-anion and dianion, respectively ... 

Also other active metal complexes (i.e., several Co SchifF base [232] and Mn diimine complexes [233]) have been supported in this way. The high dispersion of the complexes in the cages of the molecular sieves allows to study the redox properties of mononuclear complexes that are imstable in solution [234]. The increased stability of the obtained materials, the easier handling of heterogeneous catalysts and the high yields achieved make these supramolecular systems a very promising candidate for further catalyst development in fine chemical synthesis. 

Beyond these obvious roles, the spectroscopic and electrochemical signatures of metal complexes can be used to understand DNA reactivity and to detect DNA structures. In this review, efforts to exploit the redox and photophysical properties of metal complexes to understand DNA reactivity will be discussed (23, 39). Metal complexes provide a special opportunity for these studies, because they exhibit well-defined redox states that can be correlated with redox changes in nucleic acids and nucleotides. In metal complexes, changes in these redox states are coupled to changes in the optical spectroscopy of the metal center. 

The oxidation of dimethyl sulfide to the corresponding sulfoxide on different zeolites has been reported recently, using zeolite entrapped Cu-ethylenediamine ([Cu(en)2]2") complexes. Spectroscopic comparison between the neat and the NaY, KL, and NaBETA entrapped complexes, shows that the square planar complex undergoes distortion in the zeolite crystal [54-56], Changes in redox properties of the complexes in the zeolites are due to decrease of the HOMO / LUMO levels of the metal complexes upon encapsulation under influence of the electric field existing inside the zeolite [56]. The high activity in ZSM-5, however, points to the existence of extra-pore complexes, probably strongly adsorbed at the external surface. 

The nature and properties of metal complexes have been the subject of important research for many years and continue to intrigue some of the world s best chemists. One of the early Nobel prizes was awarded to Alfred Werner in 1913 for developing the basic concepts of coordination chemistry. The 1983 Nobel prize in chemistry was awarded to Henry Taube of Stanford University for his pioneering research on the mechanisms of inorganic oxidation-reduction reactions. He related rates of both substitution and redox reactions of metal complexes to the electronic structures of the metals, and made extensive experimental studies to test and support these relationships. His contributions are the basis for several sections in Chapter 6 and his concept of inner- and outer-sphere electron transfer is used by scientists worldwide. 

Charge neutralization also affects the redox properties of thioether complexes. It contributes to the marked stabilization of lower oxidation states found in all cases. Thioether complexes of metal ions in high oxidation states may approach the boundaries of the electroneutrality principle. Apart from any n-acidity of the ligands, simple electrostatic considerations suggest that poor charge neutralization by the ligands disfavors higher oxidation states. 

Taken together, the work summarized here indicates that thioethers exhibit a marked preference for the lower, softer oxidation states. Put another way, they strongly stabilize lower oxidation and spin states of metal ions. They do so by accepting electron density from the metal back into a orbitals on the thioether that are of n symmetry with respect to the metal. This delocalization manifests itself not only in the redox properties of thioether complexes, but also in their magnetic and EPR behavior. 

Bethe (1929) initiated the crystal field theory with which van Vleck, Use, and Hartman explained the color and magnetic properties of metal complexes. The crystal field theory (CFT) constitutes a fonndation for predicting the structure, stability, kinetic lability, and redox properties, and of metal complexes. It also accounts for certain trends in the physicochemical properties of metal complexes (Orgel 1952). 

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