Bond dissociation metal-halide

Table 12b. Bond dissociation enthalpies in gaseous metal halides (kJ mol-1)...

The next question which presents itself is whether we can explain why in some systems solvent co-catalysis occurs, whereas in others, apparently similar, it does not. Let it be said first that in fact there is very little experimental evidence on this point. From the thermochemical point of view one can say that alkyl halide co-catalysis is the more probable, the lower the heterolytic bond dissociation energy of the alkyl halide, the more stable the cation derived from the monomer, and the smaller the anion derived from the metal halide. It must, however, be remembered that the non-occurrence of alkyl halide co-catalysis may be due to a kinetic prohibition, i.e., an excessively high activation energy for a reaction which is thermodynamically possible. 

Average bond dissociation energies for a selection of metal halides... 

A number of reactivity studies have been performed on 6 and 8 and indicate a strongly polar (if not ionic) Mn—E bond Mn "—E,+ (E = In, Tl). Thus heterolytic bond dissociation occurs in polar ligating solvents such as MeCN or DMF, and halogens, hydrogen halides, and alkyl halides readily add across the metal-metal bond in a manner consistent with the polarity described above (13,13a,18). In the thallium example, however, the reactions are generally more complicated and result in T1(I) salts [e.g., Eq. (3)], and metal exchange reactions are also more facile, e.g., the synthesis of 6 from 8 and indium metal. In general, therefore, the chemistry of 6 and 8 is consistent with predominantly ionic behavior. 

Self-dissociation of the Lewis acid produces a situation more conducive to direct initiation, since the cationic moiety formed from the metal halide can attack the monomer more effectively and add onto its double bond to give the corresponding carboca-tion. 

Cotton and Jenkins (2) also reported bond dissociation energies for LiOH(g), KOH(g), and NaOH(g) which are in good agreement with JANAF enthalpy of formation data (7 ) for these hydroxides. Furthermore, the position of the hydroxides in the order of increasing bond dissociation energy established from their data, i.e. NaOH < KOH < RbOH < CsOH < LiOH, is that predicted by comparison with similar data on the stabilities of the alkali metal halides (8), However, trends within the bond... 

We note that Cotton and Jenkins (4) in the same paper reported bond dissociation energies for LiOH(g) and KOH(g) which are in reasonable agreement with JANAF values (5) for the enthalpy of formation of these two compounds. Furthermore, bond dissociation data for the alkali metal halides ( ) clearly establish the sodium compound as the least stable within each halide series. The potassium and sodium compounds differ by from 4.3 kcal mol for the fluorides and bromides to 3.6 kcal mol for the chlorides. 

Alkali metal halide dimers calculation of equilibrium bond distances and dissociation energies... 

The aminations of aryl halides were the first to be studied in palladium-catalyzed C—N bond formations, and are thus well developed nowadays. As the oxidative addition of the C—X bond of the aryl halide often constitutes the rate-limiting step in catalytic aminations, a relative reactivity is usually observed that is comparable to that obtained in most transition metal-catalyzed cross-couphngs. Thus, due to the decreasing bond dissociation energies, the reactivity order for hahdes is as follows Ph-Cl > Ph-Br > Ph-I [149]. 

The most common reaction of metal halides is substitution by nucleophiles to generate metal compounds containing new metal X-type ligand bonds or displacement of halide by a strong L-t5 e ligand to form a cationic species (Chapter 6). Abstraction of halide by silver salts to generate insoluble silver halides is also a common transformation of the metal-halide complexes described in Chapter 12. In polar media, halides are also known to undergo heterolytic dissociation to form solvated species. 

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