Metal-carbon cr-bonds

This chapter is concerned entirely with the insertion of carbon monoxide into transition metal-carbon cr-bonds. Sulfur dioxide insertion 154, 239), also common among transition metal-carbon complexes, will be treated in a complementary review, which is to appear later. Subject to the restrictions given at the beginning of Section VI, an attempt has been made at a complete literature coverage of the insertion of CO. Particular emphasis focuses on recent results, especially those of a kinetic and stereochemical nature. 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

Electrosynthesis and Electrochemistry of Metalloporphyrins Containing a Metal—Carbon cr-Bond... 

M. Bochmann, Organometallics 1 Complexes with Transition Metal-Carbon cr-Bonds. Oxford Univ. Press, Oxford, 1994 M. Bochmann, Organometallics 2 Complexes with Transition Metal-Carbon n-Bonds. Oxford Univ. Press, Oxford, 1994. 

The UPS of the (Me3SiCH2)4M and (Me3CCH2)4M compounds resemble those of Me4C and Me4Si above 10 eV. Below 10 eV only the s7 2 ionic state arising from ionization of the t2 metal-carbon cr-bond is discernible for the diamagnetic compounds. The Cr complexes possess two unpaired electrons. In a crystal field of Td symmetry constituted by the four R" groups the Cr(3d) electrons are expected to occupy an orbital of e symmetry consequently the... 

In retrospect, it is quite remarkable that at the same time when Chatt s group performed their studies on organometallic compounds with phosphine ligands, Dorothy Crowfoot-Hodgkin completed her work on the structure of vitamine B12, for which she was awarded the Nobel Prize for Chemistry in 1964. As far as we presently know, vitamine BJ2 is the only natural product containing a transition metal-carbon cr-bond [see P. G. Lenhert, and D. Crowfoot-Hodgkin, Structure of the 5,6-Dimethylbenzimidazolylcobamide Coenzyme, Nature 192,937-938 (1961)]. 

Insertion Reactions of Transition Metal-Carbon cr-Bonded Compounds. II. Sulfur Dioxide and Other Molecules, 12, 31... 

Three classes of transition metal-carbon cr-bonded compounds exhibit a behavior toward SO2 which is different from that discussed in Section III. They are (1) allyl, (2) propargyl, and (3) cyclopropyl and cyclo-propylmethyl complexes. Their reactions with SO2 will be considered in this same order. 

Hydrogenation Reactions Catalyzed by Transition Metal Complexes, 17, 319 Infrared Intensities of Metal Carbonyl Stretching Vibrations, 10, 199 Infrared and Raman Studies of sr-Complexes, 1,239 Insertion Reactions of Compounds of Metals and Metalloids, 5,22S Insertion Reactions of Transition Metal-Carbon cr-Bonded Compounds. I. Carbon Monoxide Insertion, 11,87... 

Main-group element or transition metal-carbon cr bonds can be cleaved by Hj. Group IVA metal alkyls, e.g., Zr, react ... 

Iridium complexes containing 1,2- and l,7-dicarba-closo-dodecarborane(12) (carb) are formed through a metal-carbon cr bond ... 

An alternative explanation for the 0.01-A elongation of metal-alkynyl C=C bonds relative to those of organic alkynes is that they are the result of metal-carbon cr-bonds that are polarized in a fashion. The... 

The NMR data for 11 displayed no evidence of any fluxional behavior for the complexes in solution. Structural features associated with the jLi-cr,fr-propenyl ligand of llh and llj are consistent with a strong metal-carbon cr bond, a typical sp -sp carbon-carbon single bond distance, and a significantly shorter olefinic carbon-carbon bond which is Tj -bonded to the second metal fragment (67,68). 

Insertions of Isocyanides into Metal-Carbon cr-Bonds... 

A. Wojcicki, Insertion Reactions of Transition Metal-Carbon cr-Bonded Compounds II. Sulfur Dioxide and Other Molecules. In Advances in Organometallic Chemistry, F. G. A. Stone and R. West, Eds., Academic Press New York, 1974, Vol. 12, pp. 31-81, and references therein. 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W(=C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon cr-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

The details of initiation mechanism are not understood. There is little doubt, however, that the reaction is a surface catalyzed process requiring the monomer to be adsorbed onto the catalyst surface. Initiation is believed to involve the formation of a metal-carbon cr-bond followed by coordination of an incoming monomer molecule and subsequent insertion into the metal-carbon bond. A mechanism proposed for the formation of the cr-bond between the metal and alkyl fragment [26] is shown in Fig. 9.8. The radicals formed in the initiation reactions may also participate in the alkylation of the transition metal ... 

Cobalt(II) and nickel(II) species reacted with XCH3 to produce intermediate species containing metal-carbon cr-bonds, a process characterised by a small positive A Vthat indicated a dissociative interchange mechanism.295,296 Similar reactions of aquated Cr(II) ions with a series of 10 different aliphatic radicals exhibited an average AV of +4.3+ 1.0cm3mol-1, also evidence for an Id mechanism.297... 

Heterolysis of the metal-carbon cr-bond in transient complexes of the type LmM"+ -R produced as shown above can proceed by one of two alternative pathways in aqueous solution.299,300 One pathway involves formally an overall oxidation of the MC and the other reduction of the MC occurs during the reaction ... 

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