Redox polynuclear complexes

An exhaustive treatment of the electrochemical behaviour of transition metal complexes is beyond the scope of this book, because the enormous number of ligands available, combined with the possibility to prepare mono- and/or polynuclear complexes using identical or mixed ligands, would render such a task almost impossible. Therefore, the discussion is limited to some aspects associated with the redox properties of (essentially) mononuclear metal complexes. In particular, we will concentrate representatively on the redox changes of first row transition metal complexes (excluding the metallocene complexes, as they have been already discussed in Chapter 4) that give stable, or relatively stable products. A systematic and useful examination of the redox activity of organometallic complexes of transition metals dated to 1984 has appeared.1... 

When interaction between the metal-based components is weak, polynuclear transition metal complexes belong to the field of supramolecular chemistry. At the roots of supramolecular chemistry is the concept that supramolecular species have the potential to achieve much more elaborated tasks than simple molecular components while a molecular component can be involved in simple acts, supramolecular species can performIn other words, supramolecular species have the potentiality to behave as molecular devices. Particularly interesting molecular devices are those which use light to achieve their functions. Molecular devices which perform light-induced functions are called photochemical molecular devices (PMD). Luminescent and redox-active polynuclear complexes as those described in this chapter can play a role as PMDs operating by photoinduced energy and electron transfer processes. ... 

The great recent development in electrochemical techniques will certainly be helpful for the study of redox processes of a metal which can occur in so many oxidation states. Multinuclear NMR spectrometers will allow increased use of 51V resonance as a routine method for the characterization of complexes in solution. Other recent developments are the study of polynuclear complexes, metal clusters (homo and hetero-nuclear) and mixed valence complexes, and it can be anticipated that these topics will soon become important areas of vanadium coordination chemistry, although the isolation of compounds with such complex... 

The charge transfers in polynuclear complexes with metal centres ready to undergo redox reactions are called intervalence charge transfer (IT). These compounds are often characterized by low-energy M-M transitions and thereby have intense colour (eg Prussian blue and its analogues). 

LMCT (X to Pt ) excitation is associated with a shift of electron density from X to Pt but finally yields Pt(II) because Pt(III) is not easily accessible. In cases that are restricted to one-electron transfer at single metal centers a simultaneous multielectron transfer can be achieved by assembling two or more metal centers in binuclear or polynuclear complexes (4). This applies, for example, to the redox couple Fe(II) and Fe(III) (PctsH2 = phthalocyaninetetrasulfonate) (6) ... 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

Polynuclear complexes, molecular dyads, triads, and other supermolecules composed of redox- and photo-active metal polypyridine units have a great promise as components of future molecular electronic or photonic devices as optical switches, relays, memories, etc. [38, 46],... 

To understand the principles of operation of polynuclear complexes and possibly improve their performance, the tunabil-ity of photophysical and redox properties has been examined in a number of such complexes. These complexes are composed of polypyridyl units of Ru and Os as chromophores and cyanide as the bridging ligand [76]. By appropriate synthetic procedures, it is possible to control the mode of coordination of the bridging cyanide to a given chromophoric unit as a nitrile (C-bonded) or isonitrile (N-bonded) [74-77]. By varying the nature of... 

The polynuclear complexes, studied in this work, give rise to redox series which are typical of systems with multiple interacting redox centres. 

The cyanide-bridged polynuclear complex Prussian Blue (PB), whose unit cell may be written as Fe4 [Fe"(CN)g] , undergoes two-step reversible redox reactions... 

A further, drastic step towards increasing structural complexity can be made by going from mononuclear to polynuclear complexes. These are systems in which two (or more) metal complex subunits are connected by one (or more) bridging ligand(s) (Fig. 1). Because of this additional complexity, the photophysics of polynuclear complexes is expected to be different from, and more interesting than, that of simple mononuclear species. This is true, in principle, also from the photocatalytic point of view, where the additional possibilities of polynuclear systems, in particular with respect to multielectron redox processes and inter-component energy or electron transfer, could find applications. 

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