Organometallic Compounds of Transition Metals

About 1830, Zeise, a Danish pharmacist, characterized a compound of stoichiometry PtCl2-C2H4 which is now known to be a dimer with chlorine bridges he also isolated, from the reaction products of chloroplatinate with ethanol, salts of the ion [C2H4PtCl3]. The structures of these ethylene com-,a H. W, Quinn and J. H. Tsai, Ado. hung. C/iem. Radiochom., 1969, 12, 217. 

It was later found that certain metal halides or ions other than Pt11, notably Cu1, Ag1, Hg11 and Pd11, formed complexes when treated with a variety of olefins. Thus cuprous chloride in aqueous suspension absorbs ethylene, both components dissolving well beyond their normal solubilities and in a 1 1 mole ratio. Solid cuprous halides also absorb some gaseous olefins, but the dissociation pressures of the complexes are quite high. The reaction of silver ions especially, with a variety of unsaturated substances, has been studied by physical measurements such as distribution equilibria between an aqueous and an organic solvent phase. The results can be accounted for in terms of equilibria of the type 

Before proceeding to a more general discussion, let us consider the structure and bonding in the two of the simplest olefin-metal complexes, the monoolefin-metal and the 1,3-diene-metal species. Several fundamental considerations applicable in more complex cases will thereby be brought out. 

Monoolefin-Metal Bonding. The basis for discussion of bonding is a clear specification of structure. The structures of two monoolefin complexes, including the anion of Zeise s salt, are shown in Fig. 23-1. The fact that the plane of the olefin, and indeed the C=C axis itself, are perpendicular to 

Donation from filled it orbitals to vacant metal orbital 

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III. Catalysts Based on the Use of Organometallic Compounds of Transition Metals. 184... 

In essence the active centers for catalytic polymerization of olefins are organometallic complexes of transition metals. For this reason a search for individual organometallic compounds that would possess catalytic activity in olefin polymerization is of great interest. The first attempts to use organometallic compounds of transition metals as catalysts for olefin polymerization were made long ago [e.g. CH3TiCl3 as a catalyst for polymerization of ethylene 116). However, only in recent years as a result of the application of relatively stable organometallic compounds of transition... 

B. Catalysts Formed by Interaction of Organometallic Compounds of Transition Metals with Oxide Supports... 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

It was later found that stable organometallic compounds of transition metals exhibiting very low polymerization activity could be transformed into high-activity catalysts when deposited on silica, alumina, or silica-alumina.287-289 Interaction of surface hydroxyl groups with the organometallic compounds such as chromocenes, benzyl, and Tt-allyl complexes results in the formation of surface-bound organometallic complexes (41-43) 289-291... 

In this chapter we will restrict our discussion of organometallic compounds to the alkyl and aryl compounds of magnesium and lithium, and the sodium and potassium salts of 1-alkynes. These substances normally are derived directly or indirectly from organohalogen compounds and are used very widely in organic synthesis. Organometallic compounds of transition metals and of boron are discussed in Chapters 11 and 31. 

Organometallic compounds of transition metals with alkyl-to-metal bonds for many years were regarded as highly unstable substances and prone to dissociate into radicals that would couple or disproportionate, as illustrated by the following sequence ... 

Dative covalent bonds, or coordinate covalent bonds, are those in which electrons are shared (as in all covalent bonds), but in which both electrons involved in each bond are contributed from the same atom. Such bonds occur in organometallic compounds of transition metals having vacant d orbitals. It is beyond the scope of this book to discuss such bonding in detail the reader needing additional information should refer to works on organometallic compounds.12 The most common organometallic compounds that have dative covalent bonds are carbonyl compounds, which are formed from a transition metal and carbon monoxide, where the metal is usually in the -1, 0, or +1 oxidation state. In these compounds the carbon atom on the carbon monoxide acts as an electron-pair donor ... 

Al. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 2nd ed. Wiley, New York, 1966. Probably the best general introduction to organo-transition metal chemistry. See Chapter 27 (pp. 719-760) Complexes of w-acceptor (w-acid) ligands Chapter 28 (pp. 761-795) Organometallic compounds of transition metals. 

Telomerization is defined as an oligomerization of dienes accompanied by addition of a heteroatom or carbon nucleophilic reagent10. It is catalyzed by various organometallic compounds of transition metals, especially palladium compounds. The nucleophiles, such as water, alcohols, amines or carboxylic acids, as well as enamines, nitroalkanes and stabilized carban-ions, are mainly introduced in the terminal position of the dimeric molecule in excellent yield10. It is also possible to direct the reaction towards an internal product functionalization. Telo-merizations with heteronucleophiles are regarded as heterocarborative addition reactions and are described in Section 1.5.8.4. 

Organogold compounds.34 Alkyl derivatives of gold were among the first organometallic compounds of transition metals to be prepared. Both gold(i) and gold(m) compounds with cr-bonds to carbon, as well as olefin complexes, are known. 

Thus, for MCMs that are pure organometallic compounds of transition metals, the most important problem is how to avoid the elimination of a metal hydride during polymerization (Eq. 4-13) [64]. Such transformations are not experimentally observed for MCMs with polymerizable groups of other types (styrene, methacrylate, etc.). 

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