A complexing agent of formula (I'): wherein: - A represents -CH 2 - or -CH(L 2 -G)- - chrom1, chrom2 are identical and are selected from the groups of formula: - chrom3 is either identical to chrom1 and chrom2, or is a group of formula: R 2, R 2 ', which may be identical or different, are selected from: H; -Alk; -phenyl; -CH 2 -CO-N-Alk; - CH 2 -CO-O-Alk; -CH 2 -CO-NH 2; -CH 2 -CO-OH; L 1, L 1 ', L 2 are selected from the divalent groups having the following formulas: - n -;-(CH 2) n -O-(CH 2) m -O-(CH 2) p -; wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5 and e is an integer from 1 to 6, preferably from 1 to 4; - E, E', which may be identical or different, are groups increasing the water-solubility of the complexing agent, selected from: -SO 3 H, -PO 2, -COOH, -N + Alk 1 Alk 2 Alk 3, a carbohydrate residue of formula -(CHOH) k -CH 2 OH, k being an integer from 3 to 12, preferably equal to 4.; - R 1, R 1 ', which may be identical or different, are selected from: -COOH, -PO R 6, R 6 being selected from the groups: phenyl, phenyl substituted with a group -SO 3 H, preferably in the ortho or para position, benzyl, methyl, ethyl, propyl, n-butyl, sec-butyl, isobutyl, tert-butyl; - G is an amine optionally protected in the form -NHBoc, a succinimidyl ester, a haloacetamide, a hydrazine, an isothiocyanate, a maleimide group, or a carboxylic acid optionally protected in the form of a group -COOMe or -COOtBu; - Alk, Alk 1, Alk 2, Alk 3, which may be identical or different, represent a (C 1 -C 6)alkyl; provided that: - when R 1 or R 1 ' represent a -COOH group, E or E' do not represent a -COOH group; - the complexing agent comprises at least one group E or -SO 3 H; - when chrom1, chrom2 and chrom3 each comprise a group R 2 or R 2 ', then A is a group -CH(L 2 -G)-in which L 2 comprises at least one group -SO 3 H or else R 6 is a phenyl group substituted with a group -SO 3 H. - A represents -CH 2 - or -CH(L 2 -G)- - chrom1, chrom2 are identical and are selected from the groups of formula: - chrom3 is either identical to chrom1 and chrom2, or is a group of formula: R 2, R 2 ', which may be identical or different, are selected from: H; -Alk; -phenyl; -CH 2 -CO-N-Alk; - CH 2 -CO-O-Alk; -CH 2 -CO-NH 2; -CH 2 -CO-OH; L 1, L 1 ', L 2 are selected from the divalent groups having the following formulas: - n -;-(CH 2) n -O-(CH 2) m -O-(CH 2) p -; wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5 and e is an integer from 1 to 6, preferably from 1 to 4; R 2, R 2 ', which may be identical or different, are selected from: H; -Alk; -phenyl; -CH 2 -CO-N-Alk; - CH 2 -CO-O-Alk; -CH 2 -CO-NH 2; -CH 2 -CO-OH; L 1, L 1 ', L 2 are selected from the divalent groups having the following formulas: - n -;-(CH 2) n -O-(CH 2) m -O-(CH 2) p -; wherein n, m, p, q are integers from 1 to 16, preferably from 1 to 5 and e is an integer from 1 to 6, preferably from 1 to 4; - E, E', which may be identical or different, are groups increasing the water-solubility of the complexing agent, selected from: -SO 3 H, -PO 2, -COOH, -N + Alk 1 Alk 2 Alk 3, a carbohydrate residue of formula -(CHOH) k -CH 2 OH, k being an integer from 3 to 12, preferably equal to 4.; - R 1, R 1 ', which may be identical or different, are selected from: -COOH, -PO R 6, R 6 being selected from the groups: phenyl, phenyl substituted with a group -SO 3 H, preferably in the ortho or para position, benzyl, methyl, ethyl, propyl, n-butyl, sec-butyl, isobutyl, tert-butyl; - G is an amine optionally protected in the form -NHBoc, a succinimidyl ester, a haloacetamide, a hydrazine, an isothiocyanate, a maleimide group, or a carboxylic acid optionally protected in the form of a group -COOMe or -COOtBu; - Alk, Alk 1, Alk 2, Alk 3, which may be identical or different, represent a (C 1 -C 6)alkyl; - when R 1 or R 1 ' represent a -COOH group, E or E' do not represent a -COOH group; - the complexing agent comprises at least one group E or -SO 3 H; - when chrom1, chrom2 and chrom3 each comprise a group R 2 or R 2 ', then A is a group -CH(L 2 -G)-in which L 2 comprises at least one group -SO 3 H or else R 6 is a phenyl group substituted with a group -SO 3 H.