A chelate comprising a lanthanide (III) ion selected from europium, terbium, samarium and dysprosium and a chelating ligand of formula (I) ** (See formula) ** in which X is selected from phenylethynyl, furyl, thienyl groups, phenyl, and pyrrole, and its substituted derivatives and wherein said substituents are selected from alkyl groups and alkoxy groups. L is a connector formed from one to ten residues, each moiety being selected from the group consisting of phenylene, alkylene containing 1-12 carbon atoms, ethynediyl (-C≡C-), ethylenediyl (-C>； = C-) , ether (-O-), thioether (-S-), amide (-CO-NH-, -CO-NR'-, -NH-CO- and -NR'-CO-), carbonyl (-CO-) , ester (-COO- and -OOC-), disulfide (-SS-), sulfonamide (-SO2-NH-, -SO2-NR'-), sulfone (-SO2-), phosphate (-O-PO2-O -), diaza (-N>； = N-), and tertiary amine, in which R 'represents an alkyl group containing less than 5 carbon atoms, and replacing L with hydrogen in any part of the initial compound, or is not present, A is a reactive group selected from isothiocyanate, bromoacetamido, iodoacetamido, maleimido, 4,6-dichloro-1,3,5-triazin-2-ylamino, pyridyldithio, thioester, aminooxy, azide, hydrazide, amino, alkyne, a methacryloyl group, carboxylic acid, acid halide, aryl ester, vinyl ester and hydroxyamine ester, ** (See formula) ** in which A 'is a group d e cleavage selected from Cl, (CH3) 2SO, H2O, and NO3 in which - is the position of the connector L and ** (See formula) * in which - is the position of the connector L, and replacing A with a hydrogen from group X when the connector L is not present, and Ra is selected from -CH2COO- and - (CH2) nN (CH2COO-) 2, where n is 2 or 3, and Rb is - (CH2) mN (CH2COO -) 2, where m is 2 or 3, and Rc is selected from CH2COO-, and - (CH2) lN (CH2COO-) 2, where l is 2 or 3.Un quelato que comprende un ion lantánido(III) seleccionado de europio, terbio, samario y disprosio y un ligando quelante de fórmula (I) **(Ver fórmula)** en la que X se selecciona de grupos feniletinilo, furi
