Cationic alkylidyne complexes

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. 

A wide range of diiron alkylidene and alkylidyne complexes have been reported over the past 10 years. In a series of papers, Ghen et aL have described the synthesis and reactivity of cationic alkylidyne complexes, [Gp2Fe2(GO)2(//-GO)(//-GAr)][BBt4] 203 (Ar = Ph, together with related SiMe2-linked cyclopentad ienyl... 

The spectroscopic properties and the reactivity towards low-valent metal fragments of these cationic alkylidyne salts mirror those of the pyrazolylborate analogues, with one notable exception being that pyrazolylmethane ligands exhibit a (protic) reactivity at the bridgehead carbon, allowing for modification of the ligand within the alkylidyne complex (Section IV.E), a phenomenon which has not been similarly documented for Tp L M( = CR) complexes (hydridic B-H bond). 

The basicity at the 3-carbon is illustrated by the reactions in Equations 13.26 and 13.27. Reaction of the octahedral rhenium vinylidene in Equation 13.26 with HBF generates the cationic carbyne complex from addition of a proton to the basic 3-carbon. Because low-valent metals are often basic, addition of a proton to the vinylidene 3-carbon is likely to occur by a multi-step process initiated by protonation of the metal center. This initial protonation at the metal center would then be followed by migration of the proton to the 3-carbon. The reaction of acid with the iridium vinylidene in Equation 13.27 illustrates this mechanism. In this case, protonation first generates an iridium-hydride complex. The hydride in this complex tlien migrates to the p-carbon to generate an alkylidyne complex. ... 

The electrophilicity of the alkylidyne carbon in cationic complexes of manganese and rhenium such as 154 or 157 is well established. A study by Chen et al. established that the carbonyl ligands are also potential sites for nucleophilic attack (157,158). Reaction of 154 with the bulky carborane anion LiCjBioH, results in the formation of two products the carbene complex 156, resulting from attack at the alkylidyne carbon, and the alkylidyne acyl complex 155, resulting from attack at a carbonyl ligand [Eq. (135)]. At room temperature complex 155 transforms into complex 156. 

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