Molecular orbital pathways complex

Of the Ru(IV) complexes recorded here most are mono-oxo species which, despite the strong axial distortion brought about by the terminal oxo ligand, are probably all paramagnetic. Semi-empirical molecular orbital calculations (INDO/1) for epoxidations effected by oxo-Ru(IV) complexes have been reported (a non-concerted [1 h- 2] pathway was preferred) [642], [643] and for alcohol oxidations by octahedral species containing an Ru" (0) unit [644]. The reactivity of high oxidation-state polypyridyl complexes of osmium and Ru, with particular emphasis on Ru(IV) and Os(IV) oxo species, has been reviewed [43]. 

After electrochemical reduction electron is placed on the lowest unoccupied molecular orbital (LUMO) of the acceptor subunits of A-D molecule. In the electrochemical oxidation, an electron is correspondingly removed from the highest occupied molecular orbital (HOMO) of the donor moiety. In the diffusion-controlled reaction electrochemically generated ions A -D and A-D+ form an activated complex A-D + A -D for which the following reaction pathways are possible ... 

Ab initio molecular orbital calculations, coupled with activation energies and entropies from experimental data, have been employed to determine the nature of the intermediates in the reaction of singlet oxygen with alkenes, enol ethers, and enamines.214 Allylic alkenes probably react via a perepoxide-like conformation, whereas the more likely pathway for enamines involves a zwitterionic cycloaddition mechanism. The reactions of enol ethers are more complex, since the relative stabilities of the possible intermediates (biradical, perepoxide, and zwitterionic) here depend sensitively on the substituents and solvent polarity. 

The alkene metathesis reaction was unprecedented - such a non-catalysed concerted four-centred process is forbidden by the Woodward-Hoffmann rules - so new mechanisms were needed to account for the products. Experiments by Pettit showed that free cyclobutane itself was not involved it was not converted to ethylene (<3%) under the reaction condition where ethylene underwent degenerate metathesis (>35%, indicated by experiments involving Di-ethylene) [10]. Consequently, direct interconversion of the alkenes, via an intermediate complex (termed a quasi-cyclobutane , pseudo-cyclobutane or adsorbed cyclobutane ) generated from a bis-alkene complex was proposed, and a detailed molecular orbital description was presented to show how the orbital symmetry issue could be avoided, Scheme 12.14 (upper pathway) [10]. 

Example 8.9. Olefin hydrogenation with Wilkinson s catalyst. Wilkinson s catalyst is a dihydrido-chloro-phosphino complex of rhodium, H2RhClPh3, where Ph is an organic phosphine such as triphenyl phosphine [48-52]. The dominant mechanism of olefin hydrogenation with this catalyst, established chiefly by Halpem [53-55] in detailed studies that included measurements of equilibria in the absence of reactants and of reaction rates of isolated participants, backed by independent NMR studies [56] and ab initio molecular orbital calculations [57], is shown as 8.69 on the facing page (without minor parallel pathways and side reactions). 

Within the past 10 years, there have been several reports on high-level molecular orbital calculations (mainly at the DFT level) being used to investigate the pathway of OA of CH3I to a variety of transition metal complexes. This work corroborates the work of Stille s group by indicating that the SN2 mechanism is the lowest energy pathway.93... 

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