14-electron spedes

This is very common espedalty for d species. A 16-electron spedes may add H2 directly, but an 18-electron spedes must lose a ligand first. A concerted H2 homolysis is often invoked, perhaps from... 

Reaction of nickelocene with methyllithium and 2-butyne leads to the formation of several organonickel complexes including the novel, 49-electron spedes [Ni3(ri -C5H5)3(MeCCCMe)], in which the alkyne bridges all three nickel atoms via two CT- and one rr-bond. ... 

The treatment of conjugated systems in terms of electron systems that extend smoothly over all atoms allows the treatment of a variety of structural phenomena, as may be explained with a spedes that shows hindered rotation and with the nitro group. 

Finally, theory involving collective electrons and the like, characteristic of ICC 1, appears less fi-equently in the catalytic hterature and theories involving local sites appear more often. The existence of coordination complex analogs may have played some part in this development. More recent theoretical developments relate primarily to chemisorbed spedes. 

Similar to usual olefins, Cjq imdergoes osmylations, epoxidations, and additions of Lewis adds (see Chapter 9). Here, Cjq acts as electron-donating spedes (Scheme 14.5). 

Allenylidenecomplexes 46 also react with thecarbon-carbon triple bond ofynamines to yield similar mononuclear cydobutenyhdene derivatives 48, although mixtures with the corresponding alkenyl-aminoallenylidene spedes 49 are formed (Scheme 2.20) [ 1 Oc]. The former isomer results from the addition of the C=C bond of y namines across the Co,=Cp unsaturation, while the latter is provided bythe formal [2 + 2] cycloaddition between C=C and Cp=Cy bonds and subsequent cycloreversion. In both processes, stepwise cyclization initiated by the addition of the nucleophilic R C=CNEt2 carbon at the Co, or Cy position, respectively, is proposed. Relative proportions of 49 with respect to 48 increase with the electron-releasing capacity of the para-substituents of the diarylallenylidene skeleton (NMe2 > OMe > Me > H). In contrast, the formation of 48 is favored when the reaction is carried out in low polarity solvents. 

Dihalobis(tertiary phosphine)nickel(II), [NiX2(PR3)2], may be either square planar or pseudotetrahedral spedes depending on the steric and electronic properties of the phosphine and, in a few cases, on the nature of the halide. 

Electron spin resonance studies of silver(II) pyridine complexes have proved to be extremely useful in determining the nature of the spedes in solution. Since natural silver has two isotopes, 107Ag and 109Ag, in approximately the same abundance, both of spin / = J, and since their nuclear magnetic moments differ by less than 15%, interpretation of spectra is often considered in terms of a single nucleus. The forms of the hyperfine splitting patterns for IN, cis and trans 2N, 3N and 4N, would be expected to be quite different and hence the number of pyridines can be readily assessed from well-resolved spectra. Spin Hamilton parameters obtained from both solid and frozen solution spectra are collected in Table 64.497 499 501-510... 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

The authors suggest that the photoexdted electron donor transfers an electron to the sulfonyl group of the tosylamine to provide an anion-radical that undergoes a heterolytic S — N bond cleavage to produce the deblocked amine after protonation as well as tosylsulfonic and sulfuric add. The function of the reducing agent is to reduce spedes such as the dimethoxybenzene cation-radical and the sulfonyl radical. 

In acetonitrile electrochemical reduction gives iron(I) and iron(O) spedes.557b In this solvent ligand oxidation reactions are almost absent, but they are not completely avoided due to the presence of small amounts of water.558 This problem was overcome by using a totally anhydrous low temperature (25 °C) molten salt comprising aluminum chloride and ethylpyridinium bromide (2 1 mole ratio). In this medium, reversible one-electron electrochemical oxidations take place559 with [Fe(37a)3]2+, [Fe(37b)3]2+ and a number of related complexes.560 Here the iron(III) form is thermodynamically more stable than is iron(II), whereas in aqueous solutions the reverse is true. 

Catalyst initiation involves the formation of a metathesis-active ruthenium species from the starting precatalyst and its entry into the catalytic cyde. For both Ru-2 and Ru-4, the initiation event consists of phosphine (PCys) dissodation to produce the 14-electron intermediate [(L)(Cl)2Rr CHR ], where L= PCys for Ru-2 and L = H2lMes for Ru-4) (Figure 6.4). Although this proposed spedes has not been observed in solution, it has been identified in the gas phase [7], and the ligand dissociation step has been studied by NMR magnetization transfer experiments. 

To achieve stability, these reactive entities may undergo any number of reactions. The purpose here is to enumerate what these reactions are and the mechanisms by which they occur. Moreover, an attempt is made to show how the organometallic free radicals are analogous to organic radicals, which are in general more familiar. The 17e organometallic species are isolobal with alkyl radicals which are 7e spedes, also one electron shy of a closed shell. 

In the light of the latter experiment, the surface peroxo species appear as a preferred target for the conduction-band electrons and, consequently, as particularly effident recombination centers. In this connection, it is to be noted that all the cathodic voltam-mograms, representing the reduction of the surface-bonded spedes photogenerated under anodic bias, exhibited significant currents already at potentials of 0.2-0.4 V versus RHE, i.e., distinctly more positive than the flat-band potential of Ti02. 

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