Metal-complexable substituted bis-indole derivatives comprising the structure shown in formula (I) hereunder: enantiomers and pharmaceutically acceptable salts thereof and metal complexes thereof, wherein L, R.sub.1, R.sub.2, R.sub.3, C.sub.1, C.sub.2, m, n, p, q and r are as defined in claim 1 for use as constrats agents.