Bonding in metal alkyls

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

On several occasions, a different model has been proposed to account for the bridge bonding in metal alkyls. In this model, it has been suggested that the bridge involved 2 atoms with an M—C—H—M interaction ... 

A15. K. Wade, Electron Deficient Compounds. Nelson, London, 1971. An excellent introduction to the reactions of and bonding in metal alkyls and carboranes. 

AHi° values of free radicals can be used to derive mean dissociation energies of metal-carbon bonds in metallic alkyls and related compounds. Thus, if both AH ° (MR , g) and AHf° (R, g) are known, the heat of the disruption process... 

The results of polymerizing ethylene using varying sigma-bonded transition metal alkyl compounds are summarized in Table VII. It is evident that none of the catalysts are very active and are comparable with the simple allyl compounds listed in Table I. 

The presence of a remote C=C double bond in the alkyl halide was critical saturated analogs underwent nickel-catalyzed halogen-zinc exchange.248 It has been suggested that the double bond coordinates to the nickel atom. As a 7r-acceptor, the C=C bond removes some electron density from the metal, thus facilitating the reductive coupling (Scheme 154).246... 

The depressed reactivity of the CO bond in metal carbonyls relative to organic carbonyls is not apparent in the case of BH3 and A1H3. For example, Masters and coworkers have observed that I B THF reduces metal acyl compounds to the corresponding alkyls, eq. 16. Although no mechanistic studies have been reported, it... 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

The Ni—C force constant is not known with certainty, but all quoted values are quite small, and are of the same order as metal-carbon force constants in metal alkyls (34). This indicates that what effect there is does not appreciably strengthen the nickel-carbon bond. Again, the effect is much greater for the anionic isosters. 

Photolysis of diethylthallium bromide in cyclohexane is a radical process involving cleavage of the thallium-carbon bond, which yields ethylcyclohexane and dicyclohexyl, as well as other products. Photoelectron transfer from benzyltributylstannanes to 10-methylacridinium ion results in cleavage of the metal-carbon bond, to give the corresponding benzyl radicals, rather than benzyl cations. Photochemical homolysis of Re- and Ru-alkyl bonds in Re(alkyl)-(CO)3(diimine) and Ru(I)(alkyl)(CO)2(diimine) complexes has been studied by Fourier transform ESR. In related manganese complexes, Mn(R)(CO)3(di-imine), elimination of CO is the predominant pathway when R = methyl, but Mn-alkyl homolysis occurs when R = benzyl. 

Metals can attain a more stable electronic configuration by accepting electrons from n-donor and n-donor systems. As discussed in Chapter 2, a variety of compounds such as alkylfithiums, aluminum, beryllium, magnesium, and other related systems form stable alkyl bridged species where the metal draws upon the electrons in a bonds in the alkyl substituent to attain electronic stability. 

The existence of hexamethyldiplatinum, (CH3)3Pt—Pt(CH3)3 (49) and the possible existence of R—Hg—Hg—R (54) are further instances. (Compare ref. 103 for possible metal-metal bonding in associated alkyls.)... 

The observation of C-H, Si-H, Si-I, and Si-Si bond activation by Pd(0) and Pt(0) complexes has prompted theoretical studies of Sakaki and coworkers [29] concerning the C-H, C-C, and Si-X (X = H, C, Si, and F) bond activation on the Pt(PH3)2 and Pd(PH3)2 complexes. In these calculations the MP2 and MP4(SDQ) methods with the double-zeta quality basis sets argumented by the polarization d function and ECP for heavier atoms have been used. As shown in Table III, the oxidative addition of C-H and C-C bonds to Pt(PH3)2 is endothermic by 6.5 and 5.2 kcal/mol, respectively, while it is exothermic by 25.6, 28.6, 14.1, and 46.4 kcal/mol for Si-H, Si-F, Si-C, and Si-Si bonds, respectively. These differences in exother-micity of the reaction have been explained in terms of the Pt-X bond energies metal-alkyl bonds are a few kilocalories per mole weaker than the metal-H bond, which in turn is 0-15 kcal/mol weaker than metal-silyl or metal-F bonds. The activation barriers increase via the trend SiH (0.7 kcal/ mol) < SiSi (17.0 kcal/mol) < SiF (26.8 kcal/mol) < SiC (28.1 kcal/mol)... 

When an alkyl group is bound to a transition metal, some of the C-H bonds in the alkyl ligand may interact with the metal. The interaction between the metal and the C-H bond at the a carbon in the alkyl group is called an a agostic interaction [6], and that between the C-H bond at the /I carbon is named a p... 

Batsanov [143-145] comprehensively revised the system of covalent radii was using the extensive structural data now available. As far as possible, the normal radii were derived directly from interatomic distances in homo-atomic molecules, elemental solids or compounds containing homo-atomic moieties. The only hetero-atomic bond distances used were the M-CH3 and M-H in metal alkyls and hydrides. 

The Ni—C bonds in nickel carbonyl are tetrahedral and of comparable strength to those in metal alkyls (28). To stabilize the coordinative Ni—C... 

Photochemical cleavage of metal-carbon bonds constitutes a brief part of a recent review on the photochemistry of metal carbonyls. Reviews of metal-alkyl and -aryl complexes have also appeared, one of which is concerned solely with metals in Groups IV— VII. The other deals with the formation, stability, and decomposition pathways of transition metal-carbon a-bonds, subjects of much interest in recent years. Consequently the recent isolation of the first stable methylene complex [Ta(Me)(CHa)(jj-C6H6)2] and its unequivocal characterization by a single-crystal JlT-ray diffraction study is of great interest in view of the probability that related derivatives are involved in the recently reported (see Vol, 4, p. 293) examples of a-hydride elimination in metal alkyls. 

The insertion of CO, NO, SO2, olefins and alkynes in metal-alkyl bonds (see Chap. 6). Dioxygen can also insert ... 

The catalytic carbonylation reaction involves CO insertion into a metal-alkyl bond. This metal-alkyl species is provided either by the oxidative addition of an organic halide RX (Monsanto process) or the insertion of an olefin into a M-H bond (0x0 process, Reppe - and related processes). We will successively examine these three industrial processes. The carbonylation reactions make a rich and varied chemistry, several examples of which are provided concerning applications to Pd(0)-catalyzed organic synthesis in Chaps 16.4 and 21. 

In metal-alkyl (R, ) bonds there is probably little contribution by ir-bonding, as the and Py orbitals of the < correct symmetry for ir-overlap with metal orbitals are already fully involved in bonding with carbon or hydrogen atoms which are also attached to this atom. 

The C-C bond cleavage of alkyl metal occurs on the jS,y-carbon-carbon bond in the alkyl group. The C-C bond cleavage of alkyl metal compounds always... 

It is commonly agreed that in metal-alkyl (Rh) bonds there is little n-contribution to the bonding. This is because the and Py orbitals of the a-carbon atom are fully involved in bonding with the carbon or hydrogens which are also attached to it. 

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