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...