Carbon-metal bonds, called anions

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

Isolated carbon-carbon double bonds are not normally reduced by dissolving metal reducing agents. Reduction is possible when the double bond is conjugated, because the intermediate anion can be stabilized by electron delocalization. The best reagent is a solution of an alkali metal in liquid ammonia, with or without addition of an alcohol - the so-called Birch reduction conditions. Under these conditions conjugated alkenes, a,p-unsaturated ketones and even aromatic rings can be reduced to dihydro derivatives. 

Furthermore, inorganic compounds present coordination geometries different from those found for carbon. For example, although 4-coordinate carbon is nearly always tetrahedral, both tetrahedral and square planar shapes occur for 4-coordinate compounds of both metals and nonmetals. When metals are the central atoms, with anions or neutral molecules bonded to them (frequently through N, O, or S), these are called coordination complexes when carbon is the element directly bonded to metal atoms or ions, they are called organometaUic compounds. 

Although only two forms of elemental carbon are common, carbon forms several anions, especially in combination with the most electropositive metals. In these compounds, called collectively the carbides, there is considerable covalent as well as ionic bonding, with the proportion of each depending on the metal. The best characterized carbide ions are shown here. 

The difference between these two kinds of carbon is one of degree, not kind. There is interaction--just how much depending on the metal and the solvent—even between electropositive ions like sodium or potassium or lithium and the anion from carbonyl compounds. These intermediates, too, could be called organometallic compounds the bonding is simply more ionic than that in, say, a Grignard reagent. 

Treatment of vinyl Sn, B, or Al compounds with BuLi results in effective addition of Bu to the metal to form a hypervalent anion such as 154. These are often referred to as ate complexes. The analogy is with the names of anions such as sulfate or carbonate. You are already familiar with the copper analogues, usually called cuprates. Lithium now replaces tin at the vinyl group 155 to form a vinyl-lithium derivative -156. The reaction is an electrophilic substitution at carbon - the lithium atom attacks the C-Sn bond and does so with retention of configuration. 

The final type of aqueous ionic equilibrium we consider involves a different kind of ion than we ve examined up to now. A simple ion, such as Na" " or S04 , consists of one or a few bound atoms, with an excess or deficit of electrons. A complex ion consists of a central metal ion covalently bonded to two or more anions or molecules, called ligands. Hydroxide, chloride, and cyanide ions are some ionic ligands water, carbon monoxide, and ammonia are some molecular ligands. In the complex ion Cr(NH3)6, for example, Cr is the central metal ion and six NH3 molecules are the ligands, giving an overall 3-1- charge (Figure 19.13). 

Potassium methide is a crystalline, ionic solid in which there are three covalent C—H bonds, and one ionic bond between the [tCHj]" anion and the K" cation. Such carbon anions, called CAR8ANI0NS, are almost invariably bonded only to the highly electropositive elements of group IR or 2R. With the less electropositive metallic elements, carbon forms a covalent bond. For example, dimethylmercury, HjC—Hg—CH, is a typical covalent compound (b.p. 96°C). Carbanions are formed as very reactive intermediates in some organic reactions. 

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