The solution behavior of (1,3-DimeU-C5)Hg(CH3COO) (1a) (1,3-DimeU = 1,3-dimethyluracil) with regard to acetate replacement by anions X (Cl-, Br-, I-, NO3-, SCN-, CN-) and by other model nucleobases (1-methylcytosine, 1-MeC, 1-methyluracil, 1-MeUH, 1-methylthymine, 1-MeTH, 9-ethylguanine, 9-EtGH, and 2-thiouracil, 2-ThioUH) has been studied, primarily by means of H-1 and Hg-199 NMR spectroscopy. Moreover, the bis(1,3-DimeU-C5) complex of Hg has been crystallized and studied by X-ray crystallography. 7a : orthorhombic system, space group Fdd2, a = 14.185(4) Angstrom, b = 25.275(7) Angstrom, c = 7.924(2) Angstrom, V = 2840(2) Angstrom(3), Z = 8. The acetato ligand of 1a is readily displaced by anions X, frequently followed by disproportionation reactions leading to HgX(2) and 7a. The donor atom X trans to C(5) has an effect on (3)J coupling between Hg-199 and H(6) of the 1,3-DimeU ligand according to NO3- > OAc- > Cl- similar to Br- > I- > SCN- > CN- > 1,3-DimeU-C5 with extremes being 222 (X = NO3-) and 107 Hz (7a). In the presence of excess metal ions (Ag+, Hg2+), 1a forms hetero- and homonuclear derivatives with the second metal ion probably sitting at O(4). The mixed nucleobase complexes have the second base bound to Hg via N(3) (1-MeU (2a), 1-MeT (3a)), N(4) (1-MeC(-) (4a), 1-MeC (4b)), N(1) (9-EtG (5a)), N(7) (9-EtGH (5b)), and N(1), N(7) (9-EtG (5c)), as well as S(2) (2-ThioU (6a)). With the exception of the 9-ethylguanine complexes 5b and 5c, all the other complexes are inert on the H-1 time scale. In several cases, e.g. 2a, 3a, 4a, and 5a, formation of dinuclear Hg or heteronuclear Ag and Pt derivatives has been established by multinuclear NMR spectroscopy.