Multiple metal carbon bonds, complexes

Finally, the possibility of building the M=C bond into an unsaturated metallacycle where there is the possibility for electron delocalization has been realized for the first time with the characterization of osmabenzene derivatives. For these reasons then, it seemed worthwhile to review the carbene and carbyne chemistry of these Group 8 elements, and for completeness we have included discussion of other heteroatom-substituted carbene complexes as well. We begin by general consideration of the bonding in molecules with multiple metal-carbon bonds. 

Acknowledgements R.R.S. thanks the National Science Foundation for supporting fundamental studies of complexes containing multiple metal-carbon bonds,... 

Metals which with adsorbed CO prefer to form metal-carbon bonds on the summits are Pt and Ir (Cu ) metals which promote binding in the valley are Pd > Ni > Rh, Re. Metals promoting multiple metal-carbon bonds (with hydrocarbons) are Ni, Ru, Rh Pt and Pd are much worse in this respect. Let us extrapolate and assume that what holds for CO also holds for hydrocarbon molecules, and that the characterization of the multiple-bond formation propensity is valid also at higher temperatures than were established experimentally by exchange reactions. Then we can attempt to rationalize the available information on the formation and the role of various hydrocarbon complexes. 

Conversion between carbyne and carbene complexes can also be carried out by methylene group transfer from Cp2Ti(jH-Cl)(j -CH2)AlMe2 to the multiple metal-carbon bond of Pt(jt-CR)W compounds. 4... 

Rupprecht, G.A., Messerle, L.W., Fellmann, J.D. and Schrock, R.R. (1980) Multiple metal-carbon bonds. 15. Octahedral alkylidene complexes of niobium and tantalum by ligand-promoted a abstraction. J. Am. Chem. Soc., 102, 6236. 

McCullough, L.G., Ustemann, M.L, Schrock, R.R., Churchill, M.R., and Ziller, J.W. (1983) Multiple metal-carbon bonds. 34. Why terminal alkynes cannot be metathesized. Preparation and crystal structure of a deproto-nated tungstacyclobutadiene complex, W(eta.5-C5H5)[Cj(CMe3)2]a./. Am. Chem. Soc., 105, 6729-6730. 

Schrock, R.R., et al. 1982. Multiple metal carbon bonds. 31. Tungsten neopentylidyne complexes. 

A number of synthetic strategies has been followed [1] for complexes with multiple metal-carbon bonds, commonly involving a nucleophilic attack, an a-abstraction or a scission process, an elimination or a rearrangement reaction at a suitable C-bonded species. 

In contrast with the extensive chemistry developed for complexes with multiple metal-carbon bonds, only very few studies of their redox properties have been reported [34],... 

The metal-carbon triple bond chemistry of ruthenium and osmium described in this article bears a close resemblance to the metal-carbon double bond chemistry of these elements as exemplified by the methylene complexes [26]. In both systems two structural classes are found, five coordinate (trigonal bipyramidal, formally zero oxidation state) and six coordinate (octahedral, formally +2 oxidation state). In both systems the five coordinate compounds exhibit multiple metal-carbon bonds which are rather non-polar and typically undergo addition reactions with electrophilic reagents. On the other hand the six coordinate compounds, both M=C and M=C, begin to show electrophilic character at the carbon centres especially in cationic complexes. Further development of the carbyne chemistry of ruthenium and osmium will depend upon the discovery of new synthetic methods allowing the preparation of a broader range of compounds with widely differing carbyne substituents. 

Following our interest on the redox properties of transition metal isocyanide and carbene complexes [1], we report the investigation of the electrochemical behaviour of new phosphonium-fiinctionalized isocyanide (A), and derived carbene (B), indole (C) and protonated indole (D) complexes of Cr, Mo and W pentacarbonyls. These studies appear to have been undertaken for the Erst time for complexes with such types of ligands. It was also our object to correlate the redox properties of these compounds with the electron donor/acceptor ability of these ligands. Moreover, this study would also extend to novel carbene complexes the rather limited electrochemical investigation reported [2] for compounds with multiple metal-carbon bonds. 

This work also gave an insight into the electrochemical behaviour of complexes with multiple metal-carbon bonds (presenting aminocarbyne, carbyne, T] vinyl, vinylidene or carbene ligands), a field which has not yet been explored in spite of the rich chemical reactivity which has already been developed for such a t e of species. Further developments are also expected, particularly in the fields of the electroactivation of the C- or N-unsaturated species and of the mechanistic investigation of their redox processes. 

Since only a small number of electrochemical studies have been reported on complexes with multiple metal-carbon bonds, and in view of our interest on such compounds, we have embarked upon the electrochemical investigation of series of carbene or carbyne complexes of Cr, Mo, W [1], Re [2], Pd or Pt [3,4]. In this work we summarize the results obtained previously [3,4] on a series of Fischer-type carbene complexes of Pd(II) and Pt(II) and report the extension of this study to further mono- and di-carbenes,... 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

The use of bulky monodentate m-terphenyl ligands in the stabilisation of d-block organometallic compounds is surveyed. Importantly, these ligands have facilitated the isolation of hitherto unknown species containing low-coordinate centres and metal-metal multiple bonds. This review reports on these advances with emphasis on the synthesis, structural characterisation and, where possible, reactivity studies of complexes featuring metal-carbon bonds between m-terphenyl ligands and the transition metals. 

As in the case of carbene complexes, 13C NMR spectroscopy is particularly useful in that the carbyne carbon typically resonates to low field (240 and 360 ppm), with heteroatom substituents shifting this to higher field. As noted above for carbene complexes, X-ray crystallography reveals that carbyne complexes have very short metal-carbon bonds, typically the shortest of any metal-carbon multiple bond, but lengthened if heteroatom substituents are present. 

Titanocene- and zirconocene-catalyzed alkene polymerization involves initial alkyl group transfer from alkylaluminum cocatalyst to Ti or Zr centers and subsequent multiple insertion of monomer into the metal-carbon bond. Zr complex catalyzed carbomagnesation shown in Eq. 5.33 [128-136] also involves alkyl ligand transfer between the main group metal and Zr. 

The insertions of alkynes into metal-carbon bonds are thermodynamically more favored than the insertions of olefins into metal-carbon bonds because the cleavage of one carbon-carbon TT-bond in an alk3me requires less energy than the cleavage of the C-C n-bond in an olefin and the sp -C-M bond in the product of alkyne insertion is stronger than the sp -C-M bond in the product of alkene insertion. The insertions of alkynes into the vinyl complexes that result from alkyne insertion are also favored thermodynamically. Thus, multiple insertions of alkynes to form polyacetylenes, just like the multiple insertions of alkenes to form polyolefins, are knoivn. Because of the conducting properties of polyacetylenes, the transition-metal-catalyzed polymerization of alkynes to form polyacetylenes has been studied. ... 

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