Carbon-metal bond dissociation

The synthetic potential of the C-Si bond cleavage from allyl- and benzylsilane radical-cations produced by photoreaction of electron-deficient iminium salts has been extensively investigated [25]. When a benzylsilane or allylsilane is irradiated in the presence of an iminium salt such as 27, a one-electron oxidation of the silane to the excited iminium salt produces a radical/radical-cation pair. Subsequent C-Si bond dissociation from the silane radical-cation by the loss of TMS+ leads to an 

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The transformation of a radical-cation into a radical involves processes such as carbon-hydrogen dissociation, carbon-metal bond dissociation, carbon-carbon bond dissociation (this latter process is not frequently encountered and will not be treated in this review) and nucleophilic addition. 

The homolytic thermal dissociation of R—M, previously used by Paneth in generating and studying free alkyl radicals, occurs with the more electronegative alkyls, such as those of mercury, lead, and the metalloids. Currently there is much interest in evaluating carbon-metal bond dissociation energies from such pyrolyscs (Section III.B). 

The temperature dependence of the extent of adsorption was not interpreted, except that the results were considered to be consistent with the magnetic measurements of Selwood (see Section II,C) which indicate that the number of carbon-metal bonds between adsorbed species and the surface increases threefold between 120°and 200°C due to extensive dissociative chemisorption. The authors proposed that two forms of chemisorbed benzene exist at the nickel surface, (i) an associatively adsorbed form which can be displaced by further benzene, and which may be w- or hexa-dissociatively adsorbed form that requires the presence of hydrogen to bring about its removal from the surface. 

It would be fitting at this stage to define in detail the various carbon species for this review, as often different terms are used in the literature. A representation of the various carbon species is shown in Figure 4.2. Surface carbide or atomic carbon can be defined as isolated carbon atoms with only carbon-metal bonds, resulting from CO dissociation or disproportionation, the latter of which is not favored on cobalt at normal FTS conditions. Recent theoretical and experimental work has indicated that the CO dissociation is preferred at the step sites, so absorbed surface carbide is expected to be located near these sites.44-46... 

In equation (2.1), De is the dissociation energy of the carbon/metal bond, which should be pretty close to the chemisorption energy of the radical-anion, as evaluated from fmite-difference calculations [7,14] a is a parameter characterizing the curvature of the Morse potential around the equihbiium position Zg. This parameter is thus connected with the vibrational frequency (Oe of the carbon/metal bond, via the reduced mass p and dissociation energy Dg of the vibrational elongation mode ... 

Addition of hydrogen to Wilkinson s catalyst promotes oxidative addition of hydrogen. Dissociation of a bulky phosphine ligand and co-ordination of the alkene is followed by stepwise stereospecific cis transfer of the two hydrogen atoms from the metal to the alkene by way of an intermediate with a carbon-metal bond (7.26). Diffusion of the saturated substrate away from the transfer site allows the released complex to combine with dissolved hydrogen and repeat the catalytic reduction cycle. 

The polynuclear carbonyl Mn2(C0)j g involves a manganese-manganese bond and five carbon-metal bonds to each of the manganese atoms, so that dissociation of the molecule into manganese atoms and carbon monoxide molecules involves breaking two different types of bonds. Only if Z)(Mn—Mn) were known would it be possible to calculate D (Mn—CO). 

This implies that the reaction will be favorable (A,G (60)<0) only if A, (60) = Z) (Me-H)— DIP (M-H) - DIP (M-Me) < —44 kJ mol, that is, the sum of metal—carbon and metal—hydrogen bond dissociation enthalpies must exceed the carbon-hydrogen bond dissociation enthalpy in methane, 439 kJ moF , by that amount. Taking a middle range value for Z)//°(M-H), 250kJmoF (Table 1), one concludes that Z) (M-Me) should be higher than ca. 230kJ moF. ... 

NHC-based catalysts feature robust carbon-metal bonds that provide high thermal stability, low dissociation rates, and, consequently, better resistance against oxidation or... 

To turn to the other extreme, numerous examples are known of heterosubstituted organometallics that spontaneously undergo -elimination. Although they obey an Elcb pattern, we again face an ambiguity. Normally the metal actively participates in the elimination process. Free carbanions are involved only in the rare cases of dissociated carbon-metal bonds (as in triphenylmethyl- or fluorenyllithiums if adequately solvated). 

Carbon monoxide eventually dissociates at room temperature on all but some of the group 8-10 (VII IB) metals (44). This dissociation occurs only for metal surfaces which form sufficiently strong metal-carbon plus metal-oxygen bonds to break the 257 kcal/mole CO bond. The known values for gas phase metal atoms predict the same trend(48). The similarity in the behavior of surfaces and atoms... 

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