Metal-carbon bond ionic

CH3 -Zn with superstoichiometric (defect) zinc atoms (Zn -impurity centres of conductivity). The larger is the electric positivity of the metal in these complexes, the larger is the ionicity of the carbon-metal bond, carbon being at the negative end of the dipole. Thus, in the case of C - K bond, ionicity amounts to 51%, whereas for C - Mg and C - Zn bonds ionicity amounts to 35% and 18%, respectively [55]. Consequently, metalloorganic compounds are characterized by only partially covalent metal-carbon bonds (except for mercury compounds). 

Recently, a deeper understanding of the precise nature of metal-carbon bonding was achieved, enabling specific polymerization catalyst systems to be designed on a practical level. The metal-carbon bond of early transition metals is partially ionic, while that of late transition metal is generally covalent. The degree of ionicity is delicately dependent on the identity of metal, formal oxidation states and auxiliary ligands. 

In view of the polarisation of the metal(+) carbon(-) bond an ionic intermediate may be expected. The retention of stereochemistry, if sometimes only temporarily, points to a concerted mechanism. 

It is desirable to examine in greater detail the reasons for the thermodynamic instability (small dissociation energy) of the alkyl carbon-transition metal a bond, which appears to be so much less than the carbon-metal a bond of the nontransition metals. The reasons for the instability are (a) the very small covalent energy of the metal-carbon bond and (6) the relatively small difference in electronegativities between the trairsition metal and the carbon atom, which accounts for the small ionic resonance energy contribution to the total energy of the bond. 

A comparison of the metal-carbon bond lengths, ionic radii and formal coordination numbers of these compounds is summarized in Table 2. The formalism used in estabhshing coordination number assumes that a -cyclooctatetraene ligand is a 5 electron-pair donor. The ionic radii have been adjusted for both the charge of the central metal and coordination number (50). When the ionic radius for a given coordination number is not available, it has been estimated by interpolation from radii of other coordination numbers. It will be seen that the differ-... 

Table 12.1 shows some organometallic lithium compounds. It is seen from their formulas that these compounds are ionic. As discussed in Section 12.2, 1A metals have low electronegativities and form ionic compounds with hydrocarbon anions. Of these elements, lithium tends to form metal-carbon bonds with the most covalent character therefore, lithium compounds are more stable (though generally quite reactive) than other organometallic compounds of group 1A metals, most... 

As discussed in Section 12.2, group 1A metals form ionic metal-carbon bonds. Organometallic compounds of group 1A metals other than lithium have metal-carbon bonds with less of a covalent character than the corresponding bonds in lithium compounds and tend to be especially reactive. Compounds of rubidium and cesium are rarely encountered outside the laboratory, so their toxicological significance is relatively minor. Therefore, aside from lithium compounds, the toxicology of sodium and potassium compounds is of most concern. 

The organometallic compound chemistry of the 2A metals is similar to that of the 1A metals, and ionically bonded compounds predominate. As is the case with lithium in group 1 A, the first 2A element, beryllium, behaves atypically, with a greater covalent character in its metal-carbon bonds. 

Sodium and potassium alkyls also polymerize methyl methacrylate to isotactic polymer at low temperatures and in hydrocarbon solvents (233, 211, 212,217). Just as with styrene, stereospecificity increases with decreasing ionic character in the metal-carbon bond and with increasing ability of the metal cation to complex monomer (K < Na< Li). 

The different behavior of the catalysts apparently arises from the nature of the transition metal of the catalyst. It seems reasonable to treat the mechanism of stereospecific olefin polymerization in terms of coordination ionic catalysts, regarding the valence state, coordination number, and nature of ligands of the transition metal as a matter of primary importance. In such an approach the polymerization mechanism is based on the character of metal—carbon bond by which a growing polymer chain is linked to the transition metal. 

Three mechanisms can be proposed for the intimate reaction mechanism for c-e, analogous to the organic 2+2 cycloadditions a pericyclic (concerted) mechanism, a diradical mechanism, and a diion mechanism. In view of the polarization of the metal(+) carbon(-) bond, an ionic intermediate maybe expected. The retention of stereochemistry, if sometimes only temporary, points to a concerted mechanism. 

The active center may be a free-radical, ion. or metal-carbon bond (Chapter 6). In any event the propagating species 4-6 will be more stable than its counterpart 4-7 if the unpaired electron or ionic charge can be delocalized across either or both substituents X and Y. When X and/or Y is bulky there will be more steric hindrance to approach of the two substituted C atoms than in attack of the active center on the methylene C as in reaction (4-1). Poly(vinyl lluoride) contains some head-to-head linkages because the F atoms are relatively small and do not contribute significantly to the resonance stabilization of the growing macroradical. 

The active center involved in the propagation reaction may be a free-radical, ion, or metal-carbon bond (see Chapters 6-10). A propagating species will be more stable if the unpaired electron or ionic charge can... 

Compounds with S and N also show pronounced differences between carbon and the other elements. Many compounds with metals are known but these are not highly ionic. Metal-carbon bonds occur in organometallic compounds. 

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