Compounds of general formula (I): with substituents as defined herein and their chelates with bi-trivalent ions of the metal elements of atomic numbers 20 to 31, 39, 42, 43, 44, 49, and 57 to 83, and radioisotopes chosen among 203Pb, 67Ga, 68Ga, 72As, 111In, 113In, 90Y, 97Ru, 62Cu, 64Cu, 52Fe, 52mMn, 140La, 175Yb, 153Sm, 166Ho, 149Pm, 177Lu, 142Pr, 159Gd, 212Bi, 47Sc, 149Pm, 67Cu, 111Ag, 199Au, 161Tb, 51Cr, 167Tm, 141Ce, 168Yb, 88Y, 165Dy, 166Dy, 97Ru, 103Ru, 186Re, 99mTc, 211Bi, 213Bi, 214Bi, 105Rh, 109Pd, 177mSn, 177Sn and 199Au, as well as the salts thereof with physiologically compatible bases or acids.