SummaryThe synthesis of [MX 3 (CO) 3 ] 2 -(M = ,88 Re, Re, "Tc) directly from the corresponding permetallates is described. Intermediates and byproducts have been isolated and characterized. Preliminary direct labeling studies at r.t. revealed a slow but irreversible incorporation of "[ 188 Re(CO) 3 ] + " into intact antibodies. The best yield was 40% after 20 h with 0.8 mg/ml of intact antibody. To elucidate possible coordination sites in proteins, the synthesis and formation of model complexes has been investigated by means of Ή NMR spectroscopy and X-ray structure analysis. A high tendency for imidazole (im) coordination has been found and is examplified in the model complexes [ReBr(histamine)-(CO) 3 ] and [Re 2^-OH) 2 (im) 2 (CO) 6 ], which structures will be presented. Additionally, complexation with the macrocyclic thioether ligand [20-aneS6] was studied in order to establish a postlabeling protocol with this type of chelator. The structure of {Tc(CO) 3 } 2 ]2+ will be presented.