Manganese carbonyl acyl complexes

While in manganese and other electronically saturated complexes an -bonded acyl (21-XXIXb) is observed as an unstable intermediate, there are numerous examples of stable V-acyls of Group 4-6 metals.153 For molybdenum a third, j8-agostic bonding mode was observed which is in equilibrium with the alkyl-carbonyl isomer 154... 

Insertion takes place between a Jt-bonded fragment and a a-bonded fragment in mutual cis-positions, as was described above. The de-insertion reaction can only proceed if there is a vacant site cis to the acyl group. The experiment outlined in Figure 2.12 proves this point. A manganese acetyl complex which is labelled with 13C at the acyl carbonyl group was synthesised and heated to give de-insertion of CO. The result was that the only product formed contained the methyl substituent in a position cis to the labelled 13C. 

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. 

It has usually been assumed that all alkyl-acyl conversions follow path (a), but there is evidence that path (b) may operate in some systems. It should also be noted that it is only because the carbonyl ligands in the manganese complexes exchange very slowly either with gaseous CO, or amongst themselves, that these experiments are possible. 

A very recent result that vinylmanganese complex was isolated by treatment of manganese acyl complex with triethylsilane gives further support for the formation of enolic species in the catalytic reaction. Carbonylation of the siloxyvinyl derivatives followed by protonation gives a-ketoacylmanganese complexes. 

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