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Carboxymethylidynetricobalt nonacarbonyl and its esters dissolve in concentrated sulfuric acid to give the acylium ion, (OC)9Co3CCO+. A stable, crystalline hexafluorophosphate salt of this acylium ion may be obtained by addition of hexafluorophosphoric acid to a solution of carboxymethylidynetricobalt nonacarbonyl or of its ethyl ester in propionic anhydride. Further reactions of this salt may be carried out by treating it directly with a nucleophilic substrate or by bringing it into reaction in the form of its slurry in dichloromethane or its solution in nitromethane. Its reactions with alcohols gave the appropriate esters: with phenyl the phenol ester, with thiols the thio esters, with ammonia and primary and secondary amines the corresponding amides, with primary amides the corresponding secondary imides, with alkylating agents such as tetramethyltin or ethylzinc bromide the corresponding ketones. The acylium hexafluorophosphate salt could be used to acylate NH2 groups of ethyl glycinate and of a tripeptide, and it acylated highly nucleophilic aromatic substrates such as JV,iV-dimethylamline, pyrrole, indole, and ferrocene. Its reduction to the aldehyde, (OC)9Co3CCHO, by triethylsilane was successful only when the reaction was carried out in the presence of an excess of aluminum chloride.

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