(1,3-Dimethyluracil-5-yl)mercury(II), (1,3-DimeU-C5)Hg-II, reacts with the model nucleobase 9-methyladenine (9-MeA) to give the mixed-nucleobase complex [Hg(1,3-DimeU-C5)(9-MeA-N6)NO3 . H2O. Hg(II) binds to the adenine via the N6 position with N1 being protonated. The neutral adenine nucleobase is therefore present in its imino form, which represents the rare tautomer form of this nucleobase. The X-ray crystal structure analysis reveals an anti orientation of the (1,3-DimeU-C5)Hg-II entity with respect to N1 of 9-MeA and substantial differences in nucleobase geometry as compared to the amino tautomer form of 9-MeA. These differences refer to both the pyrimidine and the imidazole rings of the purine base. The possible relevance of this compound with regard to nucleobase cross-linking in DNA and nucleobase mispairing is discussed. Attempts to demonstrate mispairing between the metalated rare tautomer and "the wrong" bases 1-methylcytosine (1-MeC) and 9-ethylguanine (9-EtGH) in DMSO-d(6) proved unsuccessful due to a ligand exchange process at the heavy metal.