Bridged species symmetrical

A series of pseudo-C or pseudo-C2 symmetric complexes 168-171 (Fig. 27) exhibited isotactic predominance P = 0.50-0.75) however, the isotacticity is compromised in solvent-free bulk polymerization at 130 °C [129]. Fluorous tertiary alcohol ligands with electron-withdrawing CF3 group are weakly basic and thus expected to reduce the possibility of catalyst deactivation by bridged species formation. Al complexes 172 and 173 offered highly isotactic-enriched stereoblock PLA (Pm = 0.87) from ROP of rac-lactide [168]. 

For the aluminum systems so far described, only the monomer-dimer equilibrium has been observed. For very bulkyl R groups, the monomer predominates for small alkyls, cyclopropyl, and phenyl-bridged species, the symmetrical dimer is the stable form. When the phenylethynyl group is present, this equilibrium is complicated further by the additional feature arising from the different possible types of bridge bonds, which may be described in the following manner ... 

The number of electron-deficient and normal bridge bonds remains constant throughout, but, from the very limited information available (other than by the inference from the fact that few mixed bridged species of this type have been isolated), the mixed bridged species is thermodynamically unstable with respect to the two symmetrical dimers. 

Early evidence for bridging H atoms came from nmr spectra of the cation [Cp(CO)3W]2H+, which showed satellites due to the coupling of H with both tungsten atoms symmetrically (184W, spin V2). Many bridged species, both homo- and hetero-metallic, have been confirmed by spectroscopic and diffraction methods. The main types are given in the following ... 

Mechanisms b - d all involve intramolecular migration of the group X without detachment from the two-carbon unit. The first possibility of this type (reaction b) will be referred to as the concerted mechanism and has the symmetrical bridged species as a transition structure. Such rearrangements are usually... 

Even for symmetric alkenes such as CH2=CH2 and cis-MeCH=CHMe, nons5mi-metrically bridged species are proposed. The secondary deuterium isotope effect in the intramolecularly competing methoxymercuration of CH2==CD2 ... 

The 2-butyl cation can be observed under stable-ion conditions. The NMR spectrum corresponds to a symmetrical species, which implies either very rapid hydride shift or a symmetrical H-bridged structure. 

The Creutz-Taube anion, [(NH3)5Ru- N(CH=CH)2N Ru(NH3)5] + displays more obvious redox properties, yielding both 4+ and 6- - species, and much interest has focused on the extent to which the pyrazine bridge facilitates electron transfer. A variety of spectroscopic studies supports the view that low-energy electron tunnelling across the bridge delocalizes the charge, making the 5- - ion symmetrical. Other complexes, such as the anion [(CN)5Ru (/z-CN)Ru (CN)5] , are asymmetric... 

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