Crystal structure studies indicate that metalation of the exocyclic amino group of cytosine and adenine nucleobases by Pt-II and Hg-II entities, respectively, induces protonation of a nucleobase ring nitrogen atom, and hence, causes a proton shift from an exocyclic to an endocyclic N atom. This metal-assisted process thus leads to the generation of rare nucleobase tautomers. In principle, such processes can lead to the stabilization of mispairs. The present study reports the first quantum chemical analysis of the metal-assisted tautomerization. The calculations clearly demonstrate that metalation of the exocyclic amino group of nucleobases significantly increases the protonation energy of the aromatic rings of nucleobases by about 30-34 kcal/mol for the Pt-II adduct and by about 10-14 kcal/mol for the Hg-II adduct. The calculations suggest that this kind of metalation could, besides the structural changes of DNA, significantly enhance the probability of formation of mispairs in DNA. In the course of the study, we have realized a substantial difference in terminology, which is used in computational chemistry and in bioinorganic chemistry to characterize the tautomerism of nucleobases. The difference arises since nucleobases are studied in very different environments by quantum chemical and experimental bioinorganic methods. This point is clarified and discussed in detail because it is essential for future studies of metal-assisted tautomerism of nucleobases.