Carbon metal sigma bond

It would appear from the studies on these systems that each metal atom can only accommodate one propagating center. If this were not the case, the metal atom would have attached two sigma-bonded polymer chains with hydrogen atoms on the j8-carbon atoms. The compound Zr(CH2CH2C6HB)4 has been synthesized (19) and found to be unstable above — 80°C, an observation which supports this statement. 

The purpose of this article is to review recent results on the carbonylation chemistry of actinide-to-carbon sigma bonds, bearing in mind the unique properties of 5f-organometallics cited above. We focus our attention on the properties of bis(pentamethylcyclopentadienyl) actinide acyls. Just as transition metal acyls (A) occupy a pivotal role in classical carbonylation chemistry, it will be seen that many of the unusual... 

Sigma-bond metathesis at hypovalent metal centers Thermodynamically, reaction of H2 with a metal-carbon bond to produce new C—H and M—H bonds is a favorable process. If the metal has a lone pair available, a viable reaction pathway is initial oxidative addition of H2 to form a metal alkyl dihydride, followed by stepwise reductive elimination (the microscopic reverse of oxidative addition) of alkane. On the other hand, hypovalent complexes lack the... 

Flood, Thomas C., Stereochemistry of Reactions of Transition Metal-Carbon Sigma Bonds, 12, 37. Floss, Heinz G., Stereochemistry of Biological Reactions at Proprochiral Centers, 15, 253. Freedman, T. B., Stereochemical Aspects of Vibrational Optical Activity, 17, 113. 

Green) Stereochemistry of Reactions of Transition Metal-Carbon Sigma Bonds 9 35... 

An aliphatic metal-carbon sigma bond, e. g., M-CH3, affords a fairly small C chemical shift, sometimes at negative d values. The relatively rare r allyl derivative, PdCl(q allyl)(PHOX), 61 [61], which is related to allylic alkylation chemistry, shows... 

Metal-Halogen Counpounds. One of the few examples of an olefin insertion into a metal-halogen compound has been reported by Tsuji. The reaction, which also supports the idea that sigma-bonded metal-carbon compounds are intermediates in the palladium chloride-olefin oxidation reaction, was the addition of carbon monoxide to the ethylene palladium chloride 7r-complex in nonaqueous solvents to produce a moderate yield of 3-chloropropionyl chloride (96). 

Equations 11, 12, and 13 constitute a process for the formation of the metal-carbon sigma bond. Carbonyl insertion may now occur as shown in Equation 14. 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

Flood has recently published an excellent contribution which includes a review of the stereochemistry of insertion reactions of CO and S02 into metal-carbon sigma bonds. 

A major group of organometallic compounds has carbon-metal covalent single bonds in which both the C and metal (or metalloid) atoms contribute one electron each to be shared in the bond (in contrast to ionic bonds, in which electrons are transferred between atoms). The bonds produced by this sharing arrangement are sigma-covalent bonds, in which the electron density is concentrated between the two nuclei. Since in all cases the more electronegative atom in this bond is carbon... 

The ethyl group migrates to one carbon of the double bond of the coordinated propylene, and the other carbon forms a sigma bond to the titanium. This creates a vacant coordination site on the metal, so the process can occur again. 

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