Halogens electron counting

The degree of condensation appears to be controlled by the X M ratio, not the electron count (Corbett McCarley, 1986). In a general sense, condensation appears to take place until the unit is sheathed by halogen. 

Reductive Elimination A second very common decomposition pathway for metal alkyls is reductive elimination (Eq. 3.3)/ This leads to a decrease by two units in both the electron count and the formal oxidation state. (This is why the reaction is labeled reductive. ) We study it in detail in Chapter 6. In principle it is available to all complexes, even if they are d or 18e, provided a stable oxidation state exists two units more reduced than the oxidation state in the starting alkyl. In fact, in many instances reductive elimination is not observed, for example, if X in 3.9 is a halogen. The reason is that for alkyl halides, the position of equilibrium for Eq. 3.3 usually lies well over to the side of 3.9 in other words, 3.9 is usually more stable thermodynamically. Some examples of the loss of alkyl halide are known, however. 

Second, there are other molecules, such as (X =halogens), O, RCOX, and ArX, that can also add oxidatively. Some of these reactions are of direct relevance to homogeneous catalysis and will be discussed later. Third, to be able to undergo OA, the metal complex must be electronically unsaturated, i.e., its electron count should be less than 18. All the complexes that undergo OAs in reactions 2.3.1.1- 2.3.1.3 have electron counts of 16. 

For halogenated compounds, combustion gives C02, H20, N2, and HX (X = halogen). The HX is trapped in aqueous solution and titrated with Ag+ ions in a coulometer (Section 17-3). This instrument counts the electrons produced (one electron for each Ag+) during complete reaction with HX. 

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