The present invention relates to a compound for use as a chelator in the radioactive labeling of biomolecules with metal tricarbonyl complexes, which compound has the general Formula (I) wherein M is a monovalent cation, such as Li, Na, K, Tl, Rb, Cs or an alkyl ammonium; R1 is H, alkyl, aryl or a biomolecule; R2 is H or a pendant arm, said pendant arm optionally comprises a biomolecule, with the proviso that when R1 is H, R2 is not H or COOH, and when R1 is alkyl or aryl, R2 is not H.