Metal carbon multiple bond

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,... 

This mechanism employed the postulated multiple bonds. It might be that with some metals and at higher temperatures the dehydrogenation is deeper and the multiplicity of bonds is even higher (e.g., that HC=M is also formed). In spite of this uncertainty, the multiple exchange of CH4 became a very good diagnostic tool for the 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. 

Both the long C-C bond distance (1.50 A) and the very short Pt—C distances (2.0 A) indicate the strong interaction between the adsorbed molecule and the three platinum surface atoms. The covalent Pt—C distance would be 2.2 A. The shorter metal-carbon distances indicate multiple metal-carbon bonding that may be carbene or carbyne-like. Compounds with these types of bonds exhibit high reactivity in metathesis and in other addition reactions The carbon-carbon single bond distance indicates that the molecule is stretched as much as possible without breaking of this chemical bond. 

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... 

Schrock, R.R. (2006) Multiple metal-carbon bonds for catalytic metathesis reactions (Nobel Lecture). Angew. Chem. Int. Ed., 45, 3748. 

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. 

R. R. Schrock, High Oxidation State Multiple Metal-Carbon Bonds, Chem. Rev. 102, 145-179 (2002). 

Schrock, R. R. Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture 2005). Adv. Synth. Catal. 135. 2007 349 41-53. 

I thank the chemistry division of the National Science Foundation for supporting research on multiple metal-carbon bonds for many years and the many graduate and postdoctoral students who have dedicated their time and talent to this area of research. 

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. 

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,... 

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