The present invention relates to a metal complex of the general formula M(L).sub.n, wherein each L is independently selected and represents a ligand and at least one L is vitamin B.sub.12 (cyanocobalamin) or a derivative thereof bound through the nitrogen atom of its cyanide group to M, which is an element selected from the transition metals, thus, forming a M-NC-[Co] moiety with [Co] representing vitamin B.sub.12 without cyanide and wherein n is 1, 2, 3, 4, 5 or 6. The complex can be prepared by mixing a precursor molecule with vitamin B.sub.12. The metal complexes can be used for radiodiagnostics, chemotherapy and radionuclide therapy. ##STR00001##