The invention concerns a ligand comprising (I) wherein n is an integer from 1 to 5 Y is CO2H or PO3H2T represents —X or -phenyl-X, wherein X represents NO2, NH2, NCS, NHCOCH2-Z, NHCO—W—COCNHS, —NH-Q, —NHCS-Q, —NHCOCH2-Q, or NHCO(CH2)m-Q where Q is a hapten chosen from the group consisting of steroids, enzymes, proteins, monoclonal antibodies, chimeric antibodies, or fragments thereof or any activated linker ready for coupling reaction, W is —(CH2)m- m is an integer from 1 to 10 Z is chloride, bromide or iodine ##STR1##