Coordination compounds electrochemical properties

The coordination chemistry [8] and electrochemical properties [9-11] of manganese-containing compounds have been reviewed on a number of occasions. These collections contain primarily thermodynamic and (to a lesser extent) kinetic information on compounds of relatively simple composition. The objective in this chapter is to provide a descriptive summary of the electrochemical properties of a wide range of manganese compounds. There is generous coverage of coordination complexes, which seeks to illustrate relationships between structure and... 

The coordination chemist may be interested in the electrosynthesis of compounds, the generation and detection of unstable species in unusual oxidation states and the study of their mechanisms of decay or their spectroscopic properties, or in simply obtaining thermodynamic data. These, and related topics such as using electrogenerated metallo intermediates to catalyze the transformation of inert molecules, modifying the properties of an electrode surface by adsorbing or otherwise binding a coordination compound to it, or fundamental aspects of electron-transfer kinetics, are readily studied by the application of modem electrochemical techniques. 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

The electrochemical properties of Sn(II) porphyrins in coordinating media have been discussed in a recent review [7]. The compounds were proposed to undergo two reversible one-electron macrocycle-centered reductions and an irreversible transfer of two electrons upon oxidation. The oxidation yielded ultimately a product formulated as (P)Sn (X)S]", where S = py,THFor PhCNandX = PF(s orCl04 . 

The electrochemical investigation of technetium coordination compounds has attracted considerable attention 1, 2 ] in light of the application of complexes with the isotope Tc-99m in diagnostic nuclear medicine and in view of the expected important role of the redox properties in their biological activity. 

This study provided a few examples which illustrate the application of molecular electrochemistry to the investigation of coordination compounds with unsaturated-C or -N ligands, in a number of perspectives, namely directed towards the elucidation of reaction mechanisms (either in their synthesis or in further reactions), the rationalization and prediction of their redox or electronic properties (for both the unsaturated ligands and their binding metal centres), the recognition of correlations between electrochemical and other physico-chemical parameters, and the electroinduction of chemical reactivity (electroactivation of those ligands or their coordination metal sites). 

Interpretation of electrochemical data is usually based on comparison of experimental values acquired from systematic research of similar molecules. The best situation occurs if the studied compounds form homologous series, where only one substituent is systematically changed. Then the group of substances can be treated using the linear free-energy relationship (LFER) approach [8] to analyze the role of substitution and the influence of structural modifications on redox properties [9, 10]. According to the LFER approach, reduction (oxidation) potentials of compounds belonging to the mentioned series are proportional to the experimentally determined Hammett-type constant a, which is characteristic for each substituent (special tables are available [8, 9, 11]). Recently, a theoretical treatment of substituent constants was published [12]. Eor characterization of coordination compounds, special electrochemical parameters were introduced [13]. 

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