Organometallic complexes, voltammetry

This chapter concerns the study of electrode reaction mechanisms of inorganic and organometallic complexes. The emphasis is on proper use of experimental measurables from cyclic voltammetry for diagnosis of common mechanisms such as E, EC, CE, and ECE reactions. We employ the standard designation of electron transfer (et) reactions as E, and other chemical reactions as C. In practice, mechanistic studies make use of an array of electrochemical and other physical and chemical methods, but space limitations restrict our attention to the powerful and versatile technique of cyclic voltammetry (CV). If necessary, the reader may review the fundamentals of this technique in Chapter 3. 

The organic and organometallic complexes of transition metals are especially important in catalysis and photovoltaics, on the basis of their redox and electron-mediating properties. Whilst most complex compounds can be studied in (organic) solution-phase experiments, their solid-state electrochemistry (often in an aqueous electrolyte solution environment) is in general also easily accessible by attaching microcrystalline samples to the surface of electrodes. Quite often, the voltammetric characteristics of a complex in the solid state will differ remarkably from its characteristics monitored in solution. Consequently, chemical, physical or mechanistic data are each accessible via the voltammetry of immobilized microparticles. 

TABLE 9. Cyclic voltammetry data (V) of organometallic complexes... 

Stripping voltammetry is widely used in flow approaches to determine trace levels of transition metal ions [20]. Anodic stripping voltammetry is the most exploited stripping technique for mono and multielemental determinations of metals [19,22,41,42]. Its cathodic variant and the adsorptive cathodic stripping voltammetry allow organic compounds and organometallics complexes determinations [41]. 

A key property of metallocene is their ability to exist in the form of various oxidation states with the sandwich structure and a variable number of d electrons (NVE between 14 and 20), even if the stability decreases as the NVE is further away from 18. Each metallocene exists in various oxidation states, which is usually not the case for the other families of inorganic or organometallic complexes. These various oxidation states are characterized by cyclic voltammetry, each wave corresponding to a change of oxidation state. The shape of the wave shows that, for metallocenes, this oxidation or reduction is reversible and occurs without structural change. For instance, for ferrocene, the groups of Laviron (reduction, DMF) and Bard (oxidation, SO2) have reached extreme oxidation states (Fe and Fe ). The potentials below are given V5. the saturated calomel reference electrode (SCE) ... 

A variety of vanadocene-derived organometallic species has been studied by electrochemical means. CP2VCI2 has been studied under a variety of conditions [57]. In MeCN, the oxidation appears well behaved by CV, but rotating-disk and channel-electrode voltammetry studies show that the oxidized complex undergoes follow-up chemistry shown in Eqs (1-3) ... 

All C60 adducts have low-lying LUMOs that can easily be populated by electrochemical methods. For C60 itself, six reduction couples have been observed by cyclic voltammetry (CV) or square-wave voltammetry (SWV), and as many as four reduction couples have been found for many organometallics (9,84). Most of the studies have been performed in thf or acetonitrile at lower temperatures, which increases the size of the potential window. Table VII lists the half-wave potentials for some metal complexes, and Fig. 7 shows the cyclic voltammogram for [Co(NO)(PPh3)2(i72-C60)]. 

The goal of this volume is to provide (1) an introduction to the basic principles of electrochemistry (Chapter 1), potentiometry (Chapter 2), voltammetry (Chapter 3), and electrochemical titrations (Chapter 4) (2) a practical, up-to-date summary of indicator electrodes (Chapter 5), electrochemical cells and instrumentation (Chapter 6), and solvents and electrolytes (Chapter 7) and (3) illustrative examples of molecular characterization (via electrochemical measurements) of hydronium ion, Br0nsted acids, and H2 (Chapter 8) dioxygen species (02, OJ/HOO-, HOOH) and H20/H0 (Chapter 9) metals, metal compounds, and metal complexes (Chapter 10) nonmetals (Chapter 11) carbon compounds (Chapter 12) and organometallic compounds and metallopor-phyrins (Chapter 13). The later chapters contain specific characterizations of representative molecules within a class, which we hope will reduce the barriers of unfamiliarity and encourage the reader to make use of electrochemistry for related chemical systems. 

Thiocyanate — Thiocyanate (SCN") - anions can be found in industrial wastewaters, pesticide residues, and organism metabolites. They can be determined by - ion-selective electrodes based on a variety of carriers such as organometallic compounds and metal-loporphyrin derivatives. Also, SCN- forms - complexes with metal -> ions (e.g., Ag(SCN)y) and influences the responses of metal ions in -> cyclic voltammetry. 

Beyond the ability to probe heterogeneous electron transfer reactions that occur on the micro, or even nano timescale, high-speed cyclic voltammetry has found application in the investigation of complex reaction mechanisms. An example of this is the following chemical (EC) reaction. The oxidation or reduction (E reaction) of an electroactive species can produce a chemically unstable product that undergoes a chemical reaction (C reaction) to form an electroinactive product. Electron transfer reactions can also lead to isomerizations and ligand loss in organometallic compounds, such as... 

It has been mentionned above that the binuclear thiolate complex undergoes two successive reversible oxidations. The first oxidation wave takes place at a potential more positive than the oxidation of the mononuclear complex, as generally observed in related systems. The corresponding radical is stable on the time scale of cyclic voltammetry. It could also be observed by ESR at temperatures lower than -30 C. The second oxidation leads to a cationic compound RS[Cr(CO)5]2 which can be described as an organometallic sulfonium cation isoelectronic with the previously described phosphinidene species RP[Cr(CO)5]2-... 

U. Koelle, J. Organometallic Chem., 1978, 152, 225. Cyclic voltammetry of borabenzene-cobalt complexes. 

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