The preparation of the pentacyano(iminiumacetyl)cobaltate(III) anion and its N-methyl and N,N-dimethyl derivatives is reported. The iminiumacetyl group is formed by migratory insertion of cis hydrogen isocyanide in the pentacyano(methyl)cobaltate(III) anion. The new compounds have been spectroscopically characterized by H-1, C-13, N-15, and Co-59 NMR spectroscopy and by absorption spectroscopy. The iminium carbon atoms yield C-13 NMR signals at 256.7, 247.7, and 240.4 ppm for the parent iminiumacetyl compound and its N-methyl and N,N-dimethyl derivatives, respectively. The N-15 resonance frequencies of the iminium groups and the lack of rotation of the carbon-nitrogen bond both show that this bond is best described as a double bond. The structure of (Et4N)(Ph4As)(2)[CO(CN)(5)(CH3)] was determined by X-ray crystallography at 122.0(5) K. The structure displays disorder.