Donor-acceptor complexes Subject

Electron-rich and electron-poor [3]radialenes have received much attention as components of molecular ji donor-acceptor complexes for information on this subject, see elsewhere1,21,22. 

Most importantly, the organometallic donor-acceptor complexes and their electron-transfer activated reactions discussed in this review are ideal subjects to link together two independent theoretical approaches, viz. the charge-transfer concept derived from Mulliken theory [14-16] and the free-energy correlation of electron-transfer rates based on Marcus theory [7-9]. A unifying point of view of the inner-sphere-outer-sphere distinction applies to charge-transfer complexes as well as electron-transfer processes in organometallic chemistry. 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

Despite the focus of this chapter on the most commonly utilized Lewis acids in organic synthesis, a much larger body of data regarding the structure of donor/acceptor complexes of transition metals with carbonyls exists. Although a comprehensive treatment of this subject is beyond the scope of the present discussion, it is nonetheless worthwhile to consider the structural features of some of these complexes briefly, since many demonstrate novel and unusual ways of interacting with the carbonyl group. ... 

Donor-acceptor complexes of silylated phosphoranimines Me3SiN=PR3 with a large variety of metal salts have been a continuous subject of investigations. 

I. Quantum biochemistry by A. Puixman and B. Pullman. II. Mechanisms of energy transfer by Th. FOrster. III. Char transfer in biology a) Donor-acceptor complexes in solution, (b) Transfer of charge in the organic solid state by F. J. Bullock. IV. Active transport and ion accumulation by P. Mitchell. Subject index. 

Charge-transfer materials based on donor-acceptor complexes, such as tetrathiafulvalene/tetracyanoquinodimethanide (TTF/TCNQ), once the subject of intense study as the future organic conductors, much before CPs came on the scene ... 

Relatively homogeneous fullerene LB films can be formed by mixing with arachidic acid [333]. This mixed film was used to construct a heterostructure with a PHT-arachidic acid layer, although no (bi)polaron formation was observed [341]. Donor-acceptor complexes of Cso with tetra(hexadecylthio)-TTF and BEDT-TTF were subjected to LB work. In both cases, multilayer film was formed at the air-water interface. Conductivity values of 4 and 20 S/cm for the tetra-(hexadecylthio)-TTF/C6o and the BEDT-TTF/Ceo system, respectively, were obtained after iodine doping of LB films [342]. 

Some references of reviews besides the ones already cited are given [1,3, 5-9, 19, 23-25, 28, 31, 33]. Organometallic photochemistry [36] was excellently treated in [37] and may be compared with inorganic photochemistry to gain further inspiration [38-40]. A recent multiauthored book strongly overlaps with the subject matter of the present section, and should certainly be consulted [41]. Electron transfer reactions play a determinant role in many photocatalytic processes several recent reviews and books may be cited on this topic [42-44]. The photochemistry of the M-CO bond [45] and the theme of photocatalysis by transition metal complexes [46] have recently been reviewed. Covalently linked donor-acceptor systems for mimicry of photosynthetic energy transfer have been discussed in [47]. Several special issues of Coordination Chemistry Reviews have been devoted to the photochemistry and photophysics of coordination compounds [48-50], and a special issue to photochemistry [51]. Further developments in photochemistry were the subject of a special issue of Chemical Reviews [52]. Practical considerations useful for designing photochemical experiments may be found in [53]. 

The activation of a ligand upon coordination is determined not only by the electronic properties of the binding metal site, but also by those of the ligand itself, namely its electron-donor/acceptor character. Both have a prominent effect on the redox potential of the complexes which has been the subject of parametrization methods (see also Chapter 2.19) by Pickett75 and Lever76-79 who have defined an electrochemical ligand parameter (PL or El, respectively) which is a measure of the net 7r-electron acceptor minus cr-donor ability of a ligand. 

Beyond the self-association aspect of bis(acetylacetonates) of the type M(acac)2, there has been interest in these complexes as Lewis acids, and a review has recently appeared concerning this subject 46). Complexes of the type M(acac)2 react with basic ligands such as p5uridine and water to become six-coordinate adducts. Indeed, the self-association of M(acac)2 molecules as Ni(acac)2 can be thought of in terms of inter-molecular Lewis type donor-acceptor interactions. 

Hydrogen-bonding parameters have, in part, been discussed in chapter 3.1. In addition to Seiler s Ih values [191] several other scales were derived, e.g. by Hansch and Leo (discriminating donor, acceptor, and neutral substituents) [50, 319] and by Taft (pKhb values, derived from the complex formation of bases with p-fluorophenol in CCI4) [320, 321] the subject has been reviewed in [296]. 

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