The present invention provides radioactive metal heterocomplexes of formula (I): [(Me.dbd.N--R)L.sub.1L.sub.2]+Z (I), wherein Me, R, L.sup.1 L.sup.2 and Z.sup.- have the meanings indicated in the description. The complexes include a trivalent radioactive metal-imido group, typically a technetium- or rhenium-imido group, strongly stabilized by the presence of an ancillary tridentate hetero-diphosphine ligand L.sup.1, which allows the formation of substitution-inert [(Me.dbd.N--R)L.sup.1] moieties. Such moieties are fixed in an intermediate [(Me.dbd.N--R)Y.sub.2L.sup.1)].sup.+ compound which contains two labile, cispositioned, Y ligands, where Y is preferably an halide group. The latter are easily replaced by a bidentate ligand L.sup.2 to give the final [(Me.dbd.N--R)L.sup.1L.sup.2].sup.+Z.sup.- heterocomplexes. The complexes of the invention are useful for the preparation of radiopharmaceuticals: in fact, a bioactive fragment which confers biological target-seeking properties can be introduced either on the L.sup.2 framework or the imido R group.
