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Metal-carbon bond relative strengths

Why are transition metals well suited for catalysis of this process Certainly the electrophilicity of cationic metal centers is important, as is the relative weakness of transition-metal-carbon bonds. However, similar electrophilicities and bond strengths could be found among main-group cations as well. A key to the effectiveness of Ti catalysts is the presence of two metal-based acceptor orbitals. In effect, two such orbitals are needed to choreograph the reversal of net charge flow at the two alkene carbons as the intermediate alkene complex moves through the transition state toward the final product. [Pg.518]

It can be concluded that significant differences in metal-carbon bond distances to coordinated olefins are not reflected in either the slip parameter or the barrier to rotation of the indenyl ring in this series of complexes. However, the relative strengths of the interactions between the [Rh( /5-indenyl)] fragment and OFCOT and COT are illustrated by some reactivity comparisons in Section VIII,F,2. [Pg.225]

The relative importance of a and r contributions to the overall bonding is unclear, but several different combinations of relative strengths lead to limiting case models. When there are 2 electrons in the forward (T-bond and 2 electrons in the ir-backbond, there are 2 bonding electrons for each metal-carbon bond. This is mathematically equivalent to 2tr-bonds and a metallocyclopropane structure (72). This model does not necessitate strict sp3 hybridization at the carbon atoms. Molecular orbital calculations for cyclopropane (15) indicate that the C—C bonds have higher carbon atom p character than do the C—H bonds. Thus, the metallocyclopropane model allows it interactions with substituent groups on the olefin (68). [Pg.35]

Now a close approach is possible, and interaction of several orbitals can also occur. Besides the repulsive interaction between surface c bonds, a stabilizing interaction between fragment p orbitals also takes place. This results in a relatively low activation energy of recombination that only weakly depends on the metal-carbon bond strength. Competition between C-C chain growth and meth-anation or termination (CH formation) favours C-C chain growth as the metal-carbon bond energy increases. [Pg.132]

Basically, the effect of the surface nanotexture on the strength of metal-carbon bonding may occur as a result of epitaxy or interdiffusion of atoms in the contact region of a metal crystallite and carbon support. However, information concerning these aspects of the metal-carbon interaction is scarce. Graphite-supported Pd and Pt crystallites are oriented their 202 for Pd [19] and 111 or 110 for Pt [20-22] planes parallel to the basal plane of graphite substrate, but this epitaxial interaction is relatively weak [19-21,23]. In contrast, Pd particles supported on amorphous carbons are in random orientation [19,25]. Hence, heterogeneous support surfaces comprise structurally different sites for metal-particle stabilization. [Pg.433]

Another system for which thermodynamic data have been obtained in some detail is the Tp Rh(CNneopentyl)(R)H system studied by Jones. Here, the relative thermodynamic stabilities of a number of adducts were obtained by measuring both the competitive kinetic selectivity for two types of C-H bond (AAGt in Fig. 2) as well as the barrier for reductive elimination of free alkane from each adduct (AG and AG in Fig. 2). The free energies for the latter were obtained from kinetic studies of the reductive elimination of hydrocarbon in benzene. A summary of the AG° values, calculated equilibrium constants, and relative metal-carbon bond strengths are given in Table 4 [26]. For DC H for benzene, see ref. [Pg.17]

Detailed discussion of bulk metal carbides would be inappropriate here, but aspects of their structures and thermochemistry are worth noting. Many metal carbides are metallic-type conductors of electricity, and have structures very similar to those of the bulk metals, with similar metal-metal distances, but with carbon atoms occupying interstitial sites (commonly octahedral holes) in the metal lattice. Thermochemical information is available on enough of them to get some insight into the relative strengths of both their metal-metal and metal-carbon bonding. Unfortunately, the metals that would be of most interest (osmium, rhenium, and rhodium) for the purpose of comparison with the molecular metal carbonyl carbides already discussed are not known to form stable binary carbide phases M cCj, and the carbides of the 3d metals in the same groups as these have very complicated structures. We therefore focus below on carbides of early transition metals, about which more is known. ... [Pg.174]

Another system in which metal-carbon bond strengths have been investigated in detail is the titanium system (silox)2Ti(NHSiBu 3)R. These complexes lose hydrocarbon RH to generate transient [Ti(silox)2(=NSiBu 3)], which then adds a C-H bond from the solvent across the Ti=N bond (Equation (2)). The equilibrium activation between two hydrocarbons could be directly measured in this system (AG ), and construction of a ladder of stabilities was established for some 15 substrates. In this system, relative metal carbon bond strengths could be directly obtained... [Pg.700]

Abstract The activation of C-H bonds by oxidative addition in about 30 different substrates has been examined with three closely related metal species, [Tp RhL], where L = CNneopentyl, PMes, and P(OMe)3. Kinetic studies of the reductive elimination of R-H provided data to ascertain the relative metal-carbon bond strengths for a wide range of compounds. Trends in these bond strengths reveal that there are two classes of C-H substrates parent hydrocarbons and substituted methanes. DPT calculations are used to support the observed trends, and some generalizations are made by comparison to other metal systems. [Pg.67]

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. [Pg.75]

It has been suggested that the number of cation palladium(u) also causes the isomerization of cubane to cuneane, like the silver(i) but unlike d rhodium(i) complexes, eliminates this idea. The redox properties of the respective complexes may be relevant - rhodium(i) is much readier to undergo oxidative addition than silver(i) or palladium(ii). One factor which may be of considerable importance is the relative strengths of metal-carbon bonds. It is energetically much more favourable to insert rhodium(i) into a carbon-carbon bond than to insert silver(i) into such a bond. Though the different courses of isomerizations catalysed by rhodium(i) and silver(i) complexes may be ascribed to the operation of a non-concerted mechanism for the former but... [Pg.278]

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). [Pg.3]


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See also in sourсe #XX -- [ Pg.157 , Pg.174 , Pg.175 ]




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Bond strength

Bonding carbon-metal bond

Bonding strength

Bonds carbon metal

Bonds carbon-metal bond

Carbon bond strengths

Carbon strengths

Relative bonding strength

Strength metals

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