Metal-olefin bonding, generalized

If R and R1 are not a methyl group, the process generates a chiral carbon center (C ). The overall catalytic addition of hydrogen to olefinic bonds generally is nearly always cis (7, p. 407) and to the olefinic face coordinated to the metal. This cis-endo-addition produces a chiral center(s) when one olefinic face is preferentially coordinated. 

The nature of the metal-olefin bond was studied recently in our laboratory by analyzing the natural bond orbital (NBO) results (Huang, Padin, and Yang, 1999b). The main feature of the bonding can be seen from the population changes in the vacant outer-shell s orbital of the metal and those in the d shells of the metal upon adsorption. The NBO analysis, summarized in Tables X and XI, is generally in line with the traditional picture of Dewar (1951), and Chatt and Duncanson (1953) for metal-olefin complex-ation, i.e., it is dominated by the donation and back-donation contributions, as illustrated by Fig. 13. 

The term four-electron donor is sometimes used to describe the alkyne ligand in circumstances where alkyne ir donation supplements classic metal-olefin bonding. The utility of this scheme lies in its simplicity, and with some reluctance we shall rely on the four-electron donor terminology to suggest global properties of metal alkyne monomers. The general implications and specific hazards characterizing these descriptors... 

The crystal structures of Zeise s salt and of two analogous palladium-olefin complexes were published,shortly after the publication of the Chatt-Duncanson paper. They confirmed the structural proposals made by Chatt, but none of these papers cites Dewar,though metal-olefin bonding models were not discussed. Two short reviews on the history of Zeise s salt, (one part of a more general discussion of the history of organometallic chemistry) also only refer to Chatt s contribution, though neither specifically address questions of bonding. 

Compounds containing Ad and 5d electron elements are generally more stable than those of 3d electron metals. The former compounds are characterized by greater thermal stability and by greater resistance to water and oxidizing agents. Many platinum group olefin complexes are air stable in the presence of water. This is a result of the character of the metal-olefin bond. 

In the past two chapters we have analyzed metal> olefin bonding in four different systems. Let us present a general argument which will tie this discussion together and pursue some of its ramifications. Any ML fragment has an empty orbital of a symmetry, 19.26. It interacts with the Oiled ir orbital of the olefin. There is also a filled metal orbital of (or 6i) symmetry, 19.27, available for backbonding to ethylene rr. We have taken an olefin-metal respresentation, 19.28, for these complexes. But perhaps they are viewed better as metallacyclopropane complexes, 19.29. Putting this question in another way, is there any difference be-... 

In general, it is observed that protons attached to an olefinic system which is coordinated to the metal in an olefin complex are shifted from a few tenths to about ppm to higher field from their positions in the free olefin. A qualitative estimation of the source of this shift has been given (see 27), and attributed to the difference in shielding due to the n contribution in the metal-olefin bonds, contrasted with the deshielding due to the uncoordinated 7T electrons in the free olefin. In view of the many unknown factors, this procedure was admitted to be a vast oversimplification. 

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

The polymerization of conjugated dienes with transition metal catalytic systems is an insertion polymerization, as is that of monoalkenes with the same systems. Moreover, it is nearly generally accepted that for diene polymerization the monomer insertion reaction occurs in the same two steps established for olefin polymerization by transition metal catalytic systems (i) coordination of the monomer to the metal and (ii) monomer insertion into a metal-carbon bond. However, polymerization of dienes presents several peculiar aspects mainly related to the nature of the bond between the transition metal of the catalytic system and the growing chain, which is of o type for the monoalkene polymerizations, while it is of the allylic type in the conjugated diene polymerizations.174-183... 

The kinetics of the decomposition of higher boron alkyls have not been investigated but in general these compounds undergo olefin elimination rather than metal-carbon bond rupture. 

The most efficient catalysts for the homo Diels-Alder reactions of norbornadiene were found to be cobalt327 and nickel328 complexes. The general mechanistic pathway that has been proposed for these reactions has been depicted in equation 161329. According to this mechanism, co-ordination of norbornadiene and the olefin or acetylene to the metal center gives 557, which is in equilibrium with metallocyclopentane complex 558. Then, insertion of the olefin or acetylene in the metal-carbon bond takes place to form 559. Reductive elimination finally liberates the deltacyclane species. 

In general, carbonylation proceeds via activation of a C-H or a C-X bond in the olefins and halides or alcohols, respectively, followed by CO-insertion into the metal-carbon bond. In order to form the final product there is a need for a nucleophile, Nu". Reaction of an R-X compound leads to production of equivalent amounts of X", the accumulation of which can be a serious problem in case of halides. In many cases the catalyst is based on palladium but cobalt, nickel, rhodium and mthenium complexes are also widely used. 

The polymerization of cyclic, strained olefins by transition metal alkylidenes of general formula L M = CRR (L = ligand, R, R = H, alkyl, aryl) yields polymers formed via ring-opening that contain unsaturated double bonds within each repetitive unit. Since the mechanism is based on repetitive metathesis steps, this polymerization reaction is known as ring-opening metathesis polymerization (ROMP) (Scheme 1). 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

The metal-catalysed hydrogenation of the higher olefins exhibit general features which are similar to those observed with the n-butenes. Thus, for example, the hydrogenation of hex-1-ene over Adams platinum catalyst [144] is accompanied by very low amounts of double-bond migration the relative rates of isomerisation and hydrogenation are in the ratio 0.03 1. Similarly, in the liquid phase hydrogenation of the n-pentenes over platinum—charcoal and iridium—charcoal [145], little or no isomerisation... 

Scheme 19 shows a general mechanism for C—H bond activation. In principle, any donor groups, including olefinic bonds, carbanions, heteroatom anions, neutral heteroatoms, for example, can activate their adjacent C—H bonds through coordination with appropriate transition metal centers. The metal hydride complexes formed by oxidative addition or /3-elimination, undergo unique chemical transformations. 

In general, the insertion reaction of carbon dioxide into metal hydrogen bonds is formally much akin to the analogous process involving olefins (Scheme 1). This analogy is particularly appropriate since the binding of... 

The unique bonding and the variety of structures presented by metal-olefin compounds have provided the stimulus for recent study of this class of compounds. Although many aspects of the bonding and structure of these compounds are still obscure, several general methods have been developed for their synthesis. The following series of preparations deals with the metal-olefin compounds containing more than one double bond in the hydrocarbon moiety. These preparations illustrate methods that may readily be adapted to the preparation of a number of metal-olefin compounds in which the lxydrocarbon is a diolefin. 

Over 35 years ago, Richard F. Heck found that olefins can insert into the metal-carbon bond of arylpalladium species generated from organomercury compounds [1], The carbopalladation of olefins, stoichiometric at first, was made catalytic by Tsutomu Mizoroki, who coupled aryl iodides with ethylene under high pressure, in the presence of palladium chloride and sodium carbonate to neutralize the hydroiodic acid formed (Scheme 1) [2], Shortly thereafter, Heck disclosed a more general and practical procedure for this transformation, using palladium acetate as the catalyst and tri-w-butyl amine as the base [3], After investigations on stoichiometric reactions by Fitton et al. [4], it was also Heck who introduced palladium phosphine complexes as catalysts, enabling the decisive extension of the ole-fination reaction to inexpensive aryl bromides [5],... 

There is some ambiguity as to whether the alkenyl- and alkynylsilane-forming reactions are true dehydrocoupling reactions. According to the definition as embodied in Eq. (1), this would require the direct activation of a C-H bond, such as the = C—H bond of an olefin. The general consensus is that the alkenyl- and alkynylsilane-forming reactions are more likely to take place via an insertion of the olefin into an M-Si bond, followed by /3-hydride elimination [Eqs. (18) and (19)] in the case of electron-rich metal complexes,1616 or by bond metathesis of M-Si and H -C= in the case... 

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