Reactivity and Reaction Pathways of Organometallic Compounds

It seems fair to state that the nature of the metal is more important than the R group in determining the gross behavior of R M types. Relative reactivities of organometallic compounds within a given family or horizontal period of the periodic chart have been estimated by their rate of addition to unsaturated compounds (for example, benzonitrile). The resulting empirical correlations show that R M types of the A family metals in Groups 

An additional hindrance to relating the reactivity of R M types to the electronic configuration of the metal involved is the tendency for extensive intermolecular associations of organometallic compounds. The extent to which alkyls of the boron family dimerize is a function of both the size and electric field of the metal atom. The reactivity of monomeric triethylborane versus that of dimeric triethylaluminum is therefore less readily analyzed into electronic effects. 

Conversely, a decreased reactivity of R—M types involving bulky R groups can be related to steric interference either with solvation by ether or with the substrate molecule. Such complexation seems to be important in aiding heterolysis of the carbon-metal bond. Witness the failure of f-butylmagne-sium halide to add to di-f-butyl ketone, although methylmagnesium halide adds with no special difficulty. 

In the second place, the electronic nature of the R group can modify the polarity of the carbon-metal bond. The importance of x-bonding in the structure of metal alkyls is discussed in Section III.B. As far as chemical reactivity is concerned, however, a greater electron-attracting power of R should enhance the polarity of the carbon-metal bond and thus increase its chemical reactivity. In addition, if the R group is unsaturated, the reactivity of the organometallic compound changes markedly. Situation of the unsaturation on the carbon atom of C—M bond as in phenyl, vinyl, or acetylenic R—M types usually results in some stabilization of the bond if x-metal orbital overlap can occur  

That such bonding is operative in many carbon-metal bonds is indicated by the superior stability of phenyl and acetylenic derivatives of the transition metals (Section II. E) and the reduced reactivity of vinylboranes (92, 98). Trivinylborane, for example, has a reduced acceptor tendency towards ammonia and does not react with oxygen. Investigations have aimed at looking for similar effects with vinylstannanes (112) and vinylgermanes (108). Yet in alkynylgermanes and alkynylstannanes it appears that the adjacent triple bond labilizes the carbon-metal bond. Both types undergo 

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