A complexing agent of formula: wherein: - the R 1 groups are identical and are selected from: -CO 2 H, -PO R, R being selected from the groups: phenyl, phenyl substituted by an -SO 3 H group, preferably in the ortho or para position, benzyl, methyl, ethyl, propyl, n-butyl, sec-butyl, isobutyl, tert -butyl; - the A 1, A 2 groups are identical or different and are selected from: a group of formula -L 1 -E; a group of formula or (II'); - the A 3 group is selected from: a group of formula -O-L 3 -G, a group of formula, (II'), or (III'); - L 1, L 2 and L 3 are identical or different and represent a divalent linkage group selected from: - a linear or branched C 1 -C 20 alkylene group optionally containing one or more double or triple bonds; - a C 5 -C 8 cycloalkylene group or a C 6 -C 14 arylene group, said alkylene, cycloalkylene or arylene groups optionally containing one or more hetero atoms, such as oxygen, nitrogen, sulfur, phosphorus or one or more carbamoyl or carboxamido groups, and said alkylene, cycloalkylene or arylene groups being optionally substituted by C 1 -C 8 alkyl, C 6 -C 14 aryl, sulfonate or oxo groups; - a group chosen from the divalent groups of following formulae: wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5; - E is a group that renders the complexing agent water-soluble, selected from: -SO 3 H; -PO 2; -CO 2 H; -N + Alk 1 Alk 2 Alk 3; a glucose residue in cyclic or linear form, or else a group of formula -(CHOH) k -CH 2 OH, k being an integer ranging from 3 to 12; a sulfobetaine; a PEG group of formula -CH 2 -(CH 2 OCH 2) y -CH 2 OCH 3, y being an integer ranging from 1 to 5; - G is a reactive group selected from: an acrylamide, an activated amine, an activated ester, an aldehyde, an alkyl halide, an anhydride, an aniline, an azide, an aziridine, a carboxylic acid, a diazoalkane, a haloacetamide, a halotriazine, dichlorotriazine, a hydrazine, an imido ester, an isocyanate, an isothiocyanate, a maleimide, a sulfonyl halide, a thiol, a ketone, an amine, an acid halide, a succinimidyl ester, a hydroxysuccinimidyl ester, a hydroxysulfosuccinimidyl ester, an azidonitrophenyl, an azidophenyl, a glyoxal, a triazine, an acetylenic group, and the groups of formula: wherein w varies from 0 to 8 and v is equal to 0 or 1, and Ar is a saturated or unsaturated 5- or 6-membered heterocycle, comprising 1 to 3 heteroatoms, optionally substituted by a halogen atom; - Alk 1, Alk 2, Alk 3, which may be identical or different, represent a (C 1 -C 6)alkyl. - the R 1 groups are identical and are selected from: -CO 2 H, -PO R, R being selected from the groups: phenyl, phenyl substituted by an -SO 3 H group, preferably in the ortho or para position, benzyl, methyl, ethyl, propyl, n-butyl, sec-butyl, isobutyl, tert -butyl; - the A 1, A 2 groups are identical or different and are selected from: a group of formula -L 1 -E; a group of formula or (II'); - the A 3 group is selected from: a group of formula -O-L 3 -G, a group of formula, (II'), or (III'); - L 1, L 2 and L 3 are identical or different and represent a divalent linkage group selected from: - a linear or branched C 1 -C 20 alkylene group optionally containing one or more double or triple bonds; - a C 5 -C 8 cycloalkylene group or a C 6 -C 14 arylene group, said alkylene, cycloalkylene or arylene groups optionally containing one or more hetero atoms, such as oxygen, nitrogen, sulfur, phosphorus or one or more carbamoyl or carboxamido groups, and said alkylene, cycloalkylene or arylene groups being optionally substituted by C 1 -C 8 alkyl, C 6 -C 14 aryl, sulfonate or oxo groups; - a group chosen from the divalent groups of following formulae: wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5; - a linear or branched C 1 -C 20 alkylene group optionally containing one or more double or triple bonds; - a C 5 -C 8 cycloalkylene group or a C 6 -C 14 arylene group, said alkylene, cycloalkylene or arylene groups optionally containing one or more hetero atoms, such as oxygen, nitrogen, sulfur, phosphorus or one or more carbamoyl or carboxamido groups, and said alkylene, cycloalkylene or arylene groups being optionally substituted by C 1 -C 8 alkyl, C 6 -C 14 aryl, sulfonate or oxo groups; - a group chosen from the divalent groups of following formulae: wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5; - E is a group that renders the complexing agent water-soluble, selected from: -SO 3 H; -PO 2; -CO 2 H; -N + Alk 1 Alk 2 Alk 3; a glucose residue in cyclic or linear form, or else a group of formula -(CHOH) k -CH 2 OH, k being an integer ranging from 3 to 12; a sulfobetaine; a PEG group of formula -CH 2 -(CH 2 OCH 2) y -CH 2 OCH 3, y being an integer ranging from 1 to 5; - G is a reactive group selected from: an acrylamide, an activated amine, an activated ester, an aldehyde, an alkyl halide, an anhydride, an aniline, an azide, an aziridine, a carboxylic acid, a diazoalkane, a haloacetamide, a halotriazine, dichlorotriazine, a hydrazine, an imido ester, an isocyanate, an isothiocyanate, a maleimide, a sulfonyl halide, a thiol, a ketone, an amine, an acid halide, a succinimidyl ester, a hydroxysuccinimidyl ester, a hydroxysulfosuccinimidyl ester, an azidonitrophenyl, an azidophenyl, a glyoxal, a triazine, an acetylenic group, and the groups of formula: wherein w varies from 0 to 8 and v is equal to 0 or 1, and Ar is a saturated or unsaturated 5- or 6-membered heterocycle, comprising 1 to 3 heteroatoms, optionally substituted by a halogen atom; - Alk 1, Alk 2, Alk 3, which may be identical or different, represent a (C 1 -C 6)alkyl.