Redox organometallic acceptors

Electron donor-acceptor complexes, electron transfer in the thermal and photochemical activation of, in organic and organometallic reactions, 29, 185 Electron spin resonance, identification of organic free radicals, 1, 284 Electron spin resonance, studies of short-lived organic radicals, 5, 23 Electron storage and transfer in organic redox systems with multiple electrophores, 28, 1... 

An obvious requirement for effective electron transfer between a pair of redox sites, either contained within molecules (or pairs of molecules) in solution or between a molecular species and a metal surface, is the occurrence of significant overlap between the donor and acceptor molecular orbitals. Electron-transfer reactions are unique in chemistry in proceeding even when there is extremely weak (< 1 kj mol-i) interaction between the reacting centers indeed electron transfer is believed to occur effectively over 20 - 30 A in some systems, such as metalloproteins [1]. Nevertheless, for bimolecular reactions between conventional inorganic or organometallic species, or for electrochemical processes, the reacting centers are commonly expected to be able to approach sufficiently closely (say within 4 - 8 A) prior to electron transfer so to yield substantial orbital overlap. In general, the influence of such electronic interactions upon the efficiency of electron transfer is... 

Lever s model has been applied to numerous ligands with wide ranges of electron-donor and 7r-electron-acceptor characters and binding various types of metal centers extensions thereof have also been achieved. Table 50.2 lists the values of M and 7m for a variety of metal redox couples, whereas El values for a diversity of ligands with relevance in organometallic chemistry are presented in the subsequent tables, some of them were obtained from the literature but others were estimated through application of Eq. 50.4 (see below) from the already known parameter El-... 

The redox potential of a coordination or an organometallic compound reflects its structural and electronic features, and the establishment of redox potential-structure relationships enables methods for the quantification of the net electron-donor/acceptor character of ligands and of the electronic properties of the binding metal centers. 

As ligands of intermediate hardness and Ti-acceptor power (Fig. 1.12), phosphines are able to stabilize a broad range of oxidation states and promote important catalytic reactions where redox cycling of the metal occurs in the reaction. Only cyclopentadienyl and N-heterocyclic carbenes rival phosphines in promoting organometallic catalysis. 

---