Ligand of formula wherein R 1 and R 1' each independently represents: - a hydrogen atom; or - a coupling function selected from amine; isothiocyanate; isocyanate; N-hydroxysuccinimide ester, N-hydroxyglutarimide ester, maleimide ester; carboxylic acid; acid anhydride, acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; chloroacetamide, bromoacetamide, iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide; 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; R 3 and R 3' each independently represents: - a hydrogen atom; or - an activating function selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCO R 4, wherein R 4 is selected from alkyl and aryl; provided that at least one of -L 1 -R 1 and -L 1' -R 1' represents a linker with a coupling function. R 1 and R 1' each independently represents: - a hydrogen atom; or - a coupling function selected from amine; isothiocyanate; isocyanate; N-hydroxysuccinimide ester, N-hydroxyglutarimide ester, maleimide ester; carboxylic acid; acid anhydride, acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; chloroacetamide, bromoacetamide, iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide; - a hydrogen atom; or - a coupling function selected from amine; isothiocyanate; isocyanate; N-hydroxysuccinimide ester, N-hydroxyglutarimide ester, maleimide ester; carboxylic acid; acid anhydride, acid halide; alcohol; alkyne; halide; azide; siloxy; phosphonic acid; thiol; tetrazine; norbornen; oxoamine; aminooxy; thioether; chloroacetamide, bromoacetamide, iodoacetamide; glutamate; glutaric anhydride, succinic anhydride, maleic anhydride; aldehyde; ketone; hydrazide; chloroformate and maleimide; 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; - 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; R 3 and R 3' each independently represents: - a hydrogen atom; or - an activating function selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCO R 4, wherein R 4 is selected from alkyl and aryl; - a hydrogen atom; or - an activating function selected from N-hydroxysuccinimide, N-hydroxyglutarimide, maleimide, halide and -OCO R 4, wherein R 4 is selected from alkyl and aryl; provided that at least one of -L 1 -R 1 and -L 1' -R 1' represents a linker with a coupling function.