The invention relates to a novel labeling reactant of formula (I) suitable for labeling an oligonucleotide wherein: R is a temporary protecting group. A is either a phosphorylating moiety or a solid support tethered to a bridge point Z via a linker arm E. E' is a linker arm between G and Z. G is a bivalent aromatic structure, tethered to two iminodiacetic acid ester groups N(COOR'")2 or G is a structure selected from a group consisting of or G is a protected functional group. The invention further concerns a method for direct attachment of a conjugate group to an oligonucleotide structure enabling the attachment of a desired number of these groups during chain assembly. The method comprises a Mitsonobu alkylation.