Hydrocarbon-Metal ir-Complexes

Heterocycles, inorganic, 9, 329-348 a-Heterodiazoalkanes, 9, 397-429 Homogeneous catalysis and oxidative addition reactions, 7, 84-90 Hydrocarbon-metal ir complexes, 10, 317-321... 

Hydrocarbon-Metal ir-Complexes, Other than v-Cyclopentadienyl ir-Arene Complexes 329... 

Hydrocarbon-Metal it-Complexes, Other than jt-Cyclopentadienyl and ir-Arene Complexes... 

Hydrocarbon-Metal rr-Complexes, Other than ir-Cyclopentadienyl < -ir-Arene Complexes 333... 

Hydrocarbon-Metal TT-Complexes, Other than n-Cyclopentadienyl ir-Arene Complexes 345... 

Hydrocarbon-Metal tr-Complexes, Other than Tr-Cyclopentadienyl ir-Arene Complexes 385... 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

The above type of bonding is assumed to occur in other metal-olefin and metal-acetylene complexes (172). Acetylenes have two mutually perpendicular sets of ir-orbitals and are therefore capable of being bonded to one or to two metal atoms both types of complexes are known. When the hydrocarbon is a nonconjugated polyolefin e.g., cyclo-octa-1,5-diene, each C C bond interacts independently with the metal atom. In complexes of conjugated polyolefins, e.g., cyclopentadiene, infrared and nuclear magnetic resonance studies (99) indicate that it is not yet possible to distinguish between structure (IV), in which each C C bond independently contributes two --electrons to the metal-olefin bonding, and structure (V), in which... 

Comprehensive coverage has not been attempted, but rather the intention has been to provide an introduction for the non-specialist which illustrates the scope of the field and highlights the main features of interest to organic chemists. Citation of all relevant publications is impracticable because of the large volume of literature. However, it is hoped that those key papers and specialist review articles which are included will enable the reader to locate related literature of interest. Advances in the chemistry of metallacycles and metal-hydrocarbon ir-complexes are thoroughly reviewed in surveys of organometallic chemistry published annually since 1972 (B-72MI12200). 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

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