Polynuclear complexes electronic coupling

Peculiarities of the N2 molecule make it necessary to use special means of electron transfer to and inside the active center containing the substrate. The mechanism of the catalysis in protic surroundings, at least for dinitrogen reduction, presumably necessarily includes coupled one-electron transfer from an external electron donor and multi-electron transfer to the substrate coordinated in the polynuclear complex. The coupled electron transfer helps to activate and reduce the difficult substrate dinitrogen at ambient temperatures. 

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) ... 

Electron paramagnetic resonance spectroscopy is one of the primary tools in studying the electronic structure of polynuclear complexes (341). Whereas magnetic susceptibility studies are capable of detecting electronic interactions as small as a wavenumber (discussed earlier), the EPR spectrum of a polynuclear complex may be sensitive to intramolecular exchange couplings as small as 0.001 cm even at room temperature. Additionally, the °Mn nucleus has a nuclear spin... 

Taking the imaginative, though somewhat futuristic approach out-lined in Section 1.4, some of the polynuclear complexes discussed in this article can be viewed as very simple photochemical molecular devices. Examples are the Ru(II)-Cr(III) chromophore-luminophore systems of Section 4, which perform the function of spectral sensitization (Fig 7a). The coupling of the systems for photoinduced electron transfer and charge shift described in Section 3 could lead to triads for photoinduced charge separation (Fig 7b). The trichromophoric systems described in section 5.2 can be viewed as very simple examples of the antenna effect (Fig. 7a), while the longer chain-like systems of Section 5.2 could be considered as "molecular optical fibers" suitable for remote photosensitization (Fig. 7a) and other related functions. The system described in Section 5.3, on the other hand, couples antenna effect and photoinduced electron transfer into an antenna-sensitizer function. 

The diamagnetic behavior of the diiron and tetrairon complexes, despite the presence of formally d1 and/or d9 iron centers, indicates very strong coupling between the individual paramagnetic centers all theoretical treatments of polynuclear iron-sulfur-nitrosyl complexes to date have been based on the assumption of diamagnetism in even-electron species and have employed molecular orbital methods at various levels of approximation. 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

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