New heavy transition metal carbonyl markers for protein labeling, containing an "Mn(CO)11" (M = Ru, Os, n = 3; M = Ir, n = 4) moiety, were prepared by reaction of "lightly stabilized" clusters with an N-succinimidyl ester functionalized phosphine, namely N-succinimidyl 3-diphenylphosphine-propionate (DPPS). The reaction of Os3(CO)11(DPPS) with the model amino acid beta-alanine was performed and led to the expected amide. From the reaction of Mn(CO)11(DPPS) with bovine serum albumin (BSA) in mixed organic/aqueous medium, conjugates bearing a fairly high number of metal carbonyl fragments were obtained, thus demonstrating the usefulness of this class of reagents for the selective and covalent graft of heavy metal clusters to side chain of proteins.