Electronic effects of ancillary

I.6. Electronic Effect of Ancillary Uganda on ffte Rates of Dissociative Substitution Reactions—We Cis Effect... 

Table 8.1.. The effect of ancillary ligand electronic effects on the rate of reductive elimination of benzene from Cp RhtPRjKPhKH).

Several recent studies have begun to uncover the molecular mechanisms of carbon-heteroatom bond-forming reductive elimination from Pd centers. This recent work has addressed such fundamental questions as the electronic requirements of C-X coupling, the effects of ancillary ligands, the influence of solvent and additives, and the relative rates of competing transformations. The future of this field is bright, as there are stiU many outstanding mechanistic questions to be... 

Electronic effects of the substituents on the acetylenic carbons, as well as the nature of the ancillary ligands coordinated to nickel center, also markedly affect the rate of the coupling reaction. Saito [58] compared the carboxylation rates of both Ph-C=CH and 4-MeO-C6H4-C=CH with CO2 in the presence of Ni(cod>2/DBU (2 equiv.) and found that ethynylbenzene was carboxylated faster than 4-methoxy-1-ethynylbenzene, whereas Dunach noted that the reaction rate increased with the basicity of the ancillary ligand [59]. 

The existence of critical solvation numbers for a given process to happen is an important concept. Quantum chemical calculations using ancillary solvent molecules usually produce drastic changes on the electronic nature of saddle points of index one (SPi-1) when comparisons are made with those that have been determined in absence of such solvent molecules. Such results can not be used to show the lack of invariance of a given quantum transition structure without further ado. Solvent cluster calculations must be carefully matched with experimental information on such species, they cannot be used to represent solvation effects in condensed phases. 

Complexes with (bpy)2(Cl)Ru bridged by 4,4 -bipyridine, or bipyridylethylene were prepared by Meyer et al., but unfortunately they exhibit lower couplings when compared to the analogous ruthenium(pentaammine) compounds [1, 67aj. This is attributed to the competitive effect of 7r-acceptor ancillary ligands which drain the electron density away from the bridging ligand [67b],... 

Siloxides, like alkoxides, have been employed as ancillary ligands of transition metal complexes, markedly influencing the reactivity of a metal center by electronic and steric effects of the substituents at the silicon [15, 16]. 

Mono-Cp alkoxo and aryloxo titanium thiolates and sulfides have been widely reported. These complexes show an interesting reactivity, including C-H and G-S bond activation and reduction of Ti(iv) to Ti(m). The incorporation of bulky aryloxo ligands in the coordination environment of the titanium center limits some of this reactivity and alters the chemistry of the compounds considerably. The implications of the electronic and steric effects of these ancillary ligands have been considered.1005-1008... 

Recent progress in the chemistry of structurally well-defined lanthanide ketyl and ketone dianion complexes is reviewed, with particular emphasis on the ligand effects on the reactivity of these complexes. It has been demonstrated that the stability and reactivity of the ketyl radical and ketone dianion species strongly depend on the steric and electronic properties of the ancillary ligands, the structure of their parent ketones, as well as the nature of the metals to which they are bound. Fine-tuning these factors can control the reactivity of these species. Generation and reactions of dianionic thioketone and imine species are also briefly described. 

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