Olefin-metal bonding

However, on the basis of the apparent shift in v (C=C) it has been concluded (59) that the back-bonding has the greater effect on the metal-olefin bond strength. 

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... 

Mathematical treatments of metal-olefin bonding have been given, mainly in terms of molecular-orbital theory (22, 54, 73). 

Infrared spectral studies have shown that cis- and tram-olefins are not isomerized by coordination to, or displacement from, platinum salts 127). ras-4-Methylpent-2-ene forms a more stable complex than do trans-4-methylpent-2-ene or cis-pent-2-ene it is suggested 132) that the electron-donating nature of the methyl group is responsible for the latter result by strengthening the v-component of the metal-olefin bond. 

The SCF-A a-SW calculation also confirms the importance of the back donation component of this model, and a contour plot of the relevant molecular orbital is shown in Fig. 3. Johnson and his co-workers (193) have estimated that this component may contribute up to 25% of the total metal-olefin bond energy. These calculations have given a more satisfactory account of the electronic absorption characteristics of this... 

The stabilization energy, 8E, resulting from metal-olefin bonding may be estimated from the following perturbation theory expression (220) ... 

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. 

Now that the a/3-diadsorbed species is known to be tr-complexed olefin, the simplest interpretation of rollover is that the metal-olefin bond breaks the free olefin has then a transient existence in the gas phase and can migrate from one type of site to another. That this occurs to an appreciable extent even at ambient temperatures starting with alkane in excess D2 may seem surprising but is powerful support for the olefin migration step postulated in hydrocracking and hydroreforming on dual-functional catalysts. 

Z-axis. Thus, the metal-olefin bond consists of two compounds, viz., s-donation from ligand to metal and simultaneous p-donation from metal to ligand. The metal carbon bond is, thus similar to metal-carbon bond in metal carbonyls. Both olefin and CO are weak s-donors but the presence of low energy empty p M.O. on... 

The first applications of NMR to the study of dynamic systems of the platinum group metals appear to have been studies on the rotation about the metal-olefin bond of coordinated olefins, and this process has been investigated by many workers. There are two reasonable orientations of an olefin with respect to the rest of a square planar complex, XXIV and XXV. 

Most of the ground states of complexes seem to have structure XXIV, but XXV reasonably could provide a mechanism for rotation about the metal-olefin bond axis with a low energy barrier. Cramer (II) found that 7r-cyclopentadienylbis(ethylene)rhodium(I), XXVI, gave two broad signals (r = 7.23, 8.88 ppm) for the ethylene protons at —25° and that... 

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... 

When an olefin coordinates to a transition metal, the olefin n bond donates electrons to an empty metal orbital (donor bond) and the olefin n orbital accepts metal valence electrons from a filled metal atomic orbital (back-bond). Two molecular orbitals can describe the conventional representation of the metal-olefin bond as originally proposed by Dewar and modified by Chatt 7). 

It has not been the intention of the authors to make this review a truly critical assessment of the topic. However, where appropriate, an attempt has been made to correlate the complexing tendencies of the various transition metals and to comment on the factors affecting the metal-olefin bonding. In some instances, all the metals of a group in the periodic table has been simultaneously discussed, while, in others, they have been individually reviewed. 

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