The invention relates to a labeling reactant of formula (I) useful for labeling an oligonucleotide ##STR00001## wherein: R is a temporary protecting group. A is either a phosphorylating moiety or a solid support tethered to Z via a linker arm E''. Z is a bridge point and is formed from ##STR00002## E is a linker arm between R and Z. E' is a linker arm between Z and Z'. E'' is a linker arm between Z and A. E''' is a linker arm between Z' and G. Z' is a purine or pyrimidine base. G is a protected bivalent aromatic structure, tethered to two iminodiacetic acid ester groups N(CH.sub.2COOR'').sub.2, or G is a structure selected from the group consisting of ##STR00003## or G is a protected functional group, or G is a protected or unprotected organic dye, hapten or a spin label.