A contrast agent composition and a method of diagnostic imaging are provided. The composition comprises a pharmaceutically acceptable carrier and a metal-complex comprising a ligand having structure (XXX): wherein R1, R2, R3, R7, R8, R′1, R′2, R′3, R7′ and R8′ are selected form hydrogen, a protected C1-C3 hydroxyalkyl group, or a C1-C3 alkyl group; R4, R′4 are selected from a hydrogen, a hydroxyl, a protected hydroxyl group, a protected C1-C3 hydroxyalkyl group, a C1-C3 alkyl group; n is an integer between 0 and 4; R5, R′5 are selected from a hydrogen, a protecting group comprising C1-C30 aliphatic radicals, C3-C30 cycloaliphatic radicals, C2-C30 aromatic radicals, m is an integer between 1 and 10; at least one of R7 and R′7 is acidic groups or protected acidic groups; Y comprises a protein or peptide moiety, a particle, a micelle, a liposome, an organic molecule, oligomer, polymer or a hydrophilic moiety.