Bridged species, protonated

These results indicate an energy profile for the 3-methyl-2-butyl cation to 2-methyl-2-butyl cation rearrangement in which the open secondary cations are transition states, rather than intermediates, with the secondary cations represented as methyl-bridged species (comer-protonated cyclopropanes) (Fig. 5.10). 

Several [CoL(NH3)5]3+ and m-[CoL(X)(en)2]2+ complexes have been prepared (Table 26) and some crystal structures are available (Table 25). Pentaammine complexes are easily made from [Co(DMSO)(NH3)5](C104)3, and there is no need to use a better leaving group.340,341 The physical properties ( H NMR, visible-UV, 13C NMR, pATa) for both systems (L = imH) have been comprehensively documented.341,342 The acidity of the N3 proton in (65) is enhanced by coordination and the imidazolate anion is readily accessible (pXa 10.0 vs. 14.1 for imH). Several deprotonated bimetallic bridged species have been prepared and structure (66) has been put forward as a model for heterobiometallic protein centres.343 The tetrazole complexes (68) exist predominantly in the anionic form (pA 1-2)351 and preparation normally gives the N(2)-bonded form. However Ellis and Purcell352 have prepared the N(l) isomer by reacting a coordinated nitrile with N3 ion at pH 5 (equation 52). Properties of both isomers are available.351... 

CpTi(NCMe)s]3+ by a crystal structure determination. A significant /raw-influence of the cyclopentadienyl ligand affects Ti-N bond lengths in the complex. Proton NMR studies indicate the presence of intermediate halo-bridged species in solution during successive halide abstractions 1 —> 2 3.498... 

The reverse reaction, formation of H2 from 2H+ and 2e, involves protonation of the 2Fe center to form a metal hydride. The most basic site for initial protonation in the enzyme active sites may be the electrons in the M-M bonds, which can readily be reversibly protonated to form hydride-bridged species.43 Electrophilic [M-HJ+ systems often prefer to form p-H complexes rather than M-H on deprotonation of t]2-H2 by external bases ... 

---