A chelate resulting from the complexation of a ligand of formula wherein R 1 and R 1' each independently represents: - a hydrogen atom; or - a bioactive group being di-HSGL; L 1 and L 1', each independently represents: - a single bond; or - a linker selected from alkyl, aryl, arylalkyl, alkylaryl, heteroaryl, heteroarylalkyl, alkylheteroaryl, alkenyl and alkynyl; wherein alkyl moieties are optionally interrupted by one or more heteroatoms selected from O, N and S; optionally additionally comprising a residue of a coupling function selected from amine; isothiocyanate; isocyanate; activated ester such as for example N-hydroxysuccinimide ester, N-hydroxyglutarimide ester or maleimide ester; carboxylic acid; activated carboxylic acid such as for example acid anhydride or acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; haloacetamide such as for example chloroacetamide, bromoacetamide or iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide through which R 1 and R 1' are bounded to L 1 and L 1' respectively; R 2 and R 2' each represents a hydrogen atom; L 2 and L 2' each represents a single bond; R 3 and R 3' each independently represents: - a hydrogen atom; - an activating function, wherein the activating function is selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCOR 4, wherein R 4 is selected from alkyl and aryl; or - a bioactive group being di-HSGL; provided that at least one of -L 1 -R 1 and -L 1' -R 1' represents a linker with a bioactive group being di-HSGL; with a metallic cation selected from bismuth and lead. R 1 and R 1' each independently represents: - a hydrogen atom; or - a bioactive group being di-HSGL; - a hydrogen atom; or - a bioactive group being di-HSGL; L 1 and L 1', each independently represents: - a single bond; or - a linker selected from alkyl, aryl, arylalkyl, alkylaryl, heteroaryl, heteroarylalkyl, alkylheteroaryl, alkenyl and alkynyl; wherein alkyl moieties are optionally interrupted by one or more heteroatoms selected from O, N and S; optionally additionally comprising a residue of a coupling function selected from amine; isothiocyanate; isocyanate; activated ester such as for example N-hydroxysuccinimide ester, N-hydroxyglutarimide ester or maleimide ester; carboxylic acid; activated carboxylic acid such as for example acid anhydride or acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; haloacetamide such as for example chloroacetamide, bromoacetamide or iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide through which R 1 and R 1' are bounded to L 1 and L 1' respectively; - a single bond; or - a linker selected from alkyl, aryl, arylalkyl, alkylaryl, heteroaryl, heteroarylalkyl, alkylheteroaryl, alkenyl and alkynyl; wherein alkyl moieties are optionally interrupted by one or more heteroatoms selected from O, N and S; optionally additionally comprising a residue of a coupling function selected from amine; isothiocyanate; isocyanate; activated ester such as for example N-hydroxysuccinimide ester, N-hydroxyglutarimide ester or maleimide ester; carboxylic acid; activated carboxylic acid such as for example acid anhydride or acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; haloacetamide such as for example chloroacetamide, bromoacetamide or iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide through which R 1 and R 1' are bounded to L 1 and L 1' respectively; R 2 and R 2' each represents a hydrogen atom; L 2 and L 2' each represents a single bond; R 3 and R 3' each independently represents: - a hydrogen atom; - an activating function, wherein the activating function is selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCOR 4, wherein R 4 is selected from alkyl and aryl; or - a bioactive group being di-HSGL; - a hydrogen atom; - an activating function, wherein the activating function is selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCOR 4, wherein R 4 is selected from alkyl and aryl; or - a bioactive group being di-HSGL;