Intermediates halide-bridged

Mercury(II) is known to accelerate Ligand Substitution reactions of complexes of formnla ML5X (M = Rh, Cr, Co, Ru L = H2O or NH3 X = bridging halide or psendohalide), probably by forming an intermediate with a halide bridge between Hg and M. 

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... 

Two mechanisms are possible. Dissociation of PR3 from MC1(C0)(PR3)2 (M = Rh, Ir) would result in a 14-electron three-coordinate intermediate which could then react rapidly with PR 3, or dissociation could occur from a dimeric carbonyl or halide bridged intermediate (as proposed for the halide and carbonyl exchanges). Hard evidence for neither mechanism was presented. 

The alkene inserts, as in isomerization, but the intermediate alkyl is irreversibly trapped by reductive elimination with the second hydride to give an alkane. This is an idealized mechanism. In fact, 9.4 can also solvate to give RhCl(PPh3)2(solv), and dimerize via halide bridges and each of these species have their own separate catalytic cycles that can be important under different conditions, but they all resemble Fig. 9.3. In a key study by Tolman, the dihydride was directly seen by P NMR under H2 and the reversible loss of the PPha trans to a hydride detected from a broadening of the appropriate resonance, as discussed in Section 10.5. ... 

The solid anhydrous halides of some of the transition metals are often intermediate in character between ionic and covalent their structures are complicated by (a) the tendency of the central metal ion to coordinate the halide ions around it, to form an essentially covalent complex, (b) the tendency of halide ions to bridge, or link, two metal ions, again tending to covalency (cf. aluminium chloride, p. 153 and iron(III) chloride, p. 394). 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Simple 1,2-additions to this compound have been observed123131132 also in other sulfenylation reactions, and in other electrophilic additions involving strongly bridged intermediates. Although these results have been interpreted as evidence that additions of sulfenyl halides to symmetrical alkenes do not involve open carbenium ions before the product-determining step, the different behavior observed in the case of 49 suggests123 that close proximity is necessary to have transannular participation of 7r-bonds, at least in additions of sulfenyl derivatives and of some other electrophiles carried out in the presence of efficient nucleophiles. 

Areneselenenyl halides react with double bonds similarly to sulfenyl derivatives 1,2-additions are generally anti stereospecific, in agreement with the involvement of a bridged intermediate [episelenurane (a) and/or seleniranium ions (b)], prior to the product-forming step. 

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