Metal—carbon triple bonds electrophiles, reactions with

The reagent [MejSSMe] is capable of transferring a [SMe] group to metal-carbon triple bonds [see also Eq. (118)] 142). The reaction may be viewed as a direct electrophilic attack at the MC n bond. With complex 128 the dithiocarbene complex 129 is obtained [Eq. (112)]. Complex 129 reacts with nucleophiles (H , CHj", and PMe3), which add to the carbene carbon to give stable adducts [Eq. (113)]. The carbonylmetallate anions... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

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. 

Carbynes, (RCj), are 2e ligands and their carbon atoms have sp hybridization (two p orbitals are not hybridized). Therefore, in carbyne complexes L M —CR, two molecular orbitals d -p are formed. Thus, formally, the metal-carbon bond is triple. Calculations of electronic structures of [(OC)5Cr = CH] and [(OC)4ClCr=CH] show that the carbyne carbon charge is negative. Because in cationic carbyne complexes the carbon atom reacts with nucleophiles, this carbon s reactivity is not controlled by the charge, contrary to these calculations. The contradictions between calculations and experiments may be explained " by means of the frontier orbital theory (reactions controlled by frontier orbitals). Nucleophiles react with the atom of a molecule on which the LUMO is located, while electrophiles react with the atom of a molecule on which the HOMO is mainly localized. In the case of [(OC)5Cr = CH], the maximal coefficient in LUMO is at the carbyne carbon atom, and therefore it reacts with the nucleophile despite the fact that it is negative. 

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