New specific reagents for the conjugation of organo transition metal species to proteins are described. These reagents are pyrylium salts bearing a (eta 5-C5H4)M(CO)3 (M = Mn and Re) at position 4. They couple with simple amines (n-butylamine and tert-butylamine) and to lysine side chains of proteins (bovine serum albumin and lysozyme) with varying yields. In almost all cases, the final conjugated species is a pyridinium salt, with the exception of lysozyme, for which the reaction ends at the divinylogous amide form. Differences in reactivity for bovine serum albumin and lysozyme can be explained in terms of differences of isoelectric point and steric local environment around the reactive lysine residue.