Olefin transition metal complexes stability

Transition metal complexes act as templates that regulate organic reactions that occur in the coordination sphere (4). Ligands are often activated or stabilized by participation of metal d orbitals, where the central metals are electronically amphoteric in contrast to the main group elements, which normally act as Lewis acids. The bonding scheme of an olefin-transition metal complex is illustrated in Scheme 2. The olefin 7T electrons are donated to a vacant metal orbital to make a a-type bond the metal d elections are back-donated to olefin anti-bonding orbitals with the same symmetry to form a ir-type bond. In this way, the olefin is activated by formal electron promotion from the tt to tt orbital, as... 

Transition metal hydrides play a key role in the catalytic homogeneous isomerization of olefins. The pure hydrides such as HCo(CO)4 can function as the catalyst, or transition metals complexed to stabilizing ligands can function as catalysts the catalysis almost certainly proceeds through hydride intermediates in many cases. 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

Carbenes can be stabilized as transition metal complexes decomposition of phenyldiazomethane in the presence of a ruthenium(II) complex gives a carbene complex stable enough to be isolated and stored for months. These complexes are among the most important of carbene-derived reagents because of a remarkable reaction known as alkene (or more commonly olefin) metathesis. 

Strictly speaking, a catalyst is some species directly involved in the catalytic cycle and, in the reactions discussed here, these species are usually low-valent, coordinatively unsaturated transition metal complexes. Metal halides, e-.g., chloroplatinic acid, PdCl, etc., although often claimed as catalysts are more properly catalyst precursors, since in the presence of silyl hydrides the metal halides are reduced. If no stabilizing ligands, e.g., olefins, phosphines, etc. are present, the reduction normally proceeds to a finely divided form of the metal or to insoluble metal silyl/hydride clusters which may act as heterogeneous catalysts. 

In conclusion, electronic density of the transition metal may be influenced, case by case, by the effect of the reaction with aluminum alkyl and, as a result, the carbon-transition metal bond stability, olefin coordination and insertion capacity, stereochemical control of active centre and chain transfer and propagation processes, hence polymer MWD, may also be affected. This is particulary true for soluble catalytic systems for which the existence of active centres as bimetallic complexes is likely. 

Upon the addition of CO or H2 in the presence of appropriate stabilizers, the controlled chemical decomposition of zerovalent transition metal complexes yields isolable products in multigram amounts [49]. The growth of metallic Ru particles from Ru(COT) (COD) (COT = cyclooctatetraene, COD = cycloocta-1,5-diene) with low-pressure dihydrogen was first reported by Ciardelli et al. [49a]. This material was, however, not well characterized, and the colloidal aspect of the ill-defined material seems to have been neglected in this work. Bradley and Chaudret [49b-l] have demonstrated the use of low-valent transition metal olefin complexes as a very clean source for the preparation of nanostructured mono- and bimetallic colloids. 

It is appropriate at this point to indicate our personal motivation for carrying out structural studies, the types of compounds we study, and the experimental conditions we employ. In a very general sense we are interested in the bonding of small molecules and ions, e.g., 02, N2, NO, N2 R+, olefins, and acetylenes, to transition-metal complexes. Because of our interest in bonding, we seek the best solutions we can attain. Rapid, qualitative answers to conformational problems are not our interest. Since those transition-metal systems that bind small molecules generally have the metal in a low oxidation state, and since a low oxidation state is usually stabilized by ligands of the type PR3 (R = alkyl or aryl), solution to our problems involves typically the determination of a large number of structural parameters. With only a few exceptions the intensity data are obtained at room temperature on a Picker FACS-1 computer-controlled diffractometer. Usually the ratio of observations to variables is at least 10, and it is often 20 to 30. 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

Pis serve as catalysts for transition metal complexes. These metal eom-plexes are suitable for the epoxidation of olefins. The heterogeneous PI-supported transition metal complex catalysts provide superior catal5dic activity, selectivity and stability in the epoxidation of higher olefin. Because of the heterogeneous nature, the catalysts can be easily separated from the reaction product, which eases recycling of the catalysts." ... 

An active catalyst site requires a metal-carbon bond that may have existed in the pre-catalyst, may have been formed upon initial activation by cocatalyst (via ligand exchange), or may exist because of a previous migratory insertion event. In most cases, the starting precursor of the catalyst is a metallocene dichloride (dichlorides are usrraUy the most artive precursors for coordination polymerization) complex, which obtains a vacant site as a consequence of reaction with cocatalyst (see Section 3.21.3.1 below). In the case of metallocene activation by MAO, the produced active center is a strongly Lewis acidic cationic metal complex stabilized by a bulky MAO anion the transition metal bears a vacant coordination site ready for complexation of the olefinic monomer (Figure 4(a)). 

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