A systematic quantum-chemical characterization of intrinsic structural and energetical properties of the model nucleobase complex trans-[Pt(NH3)(2)(CYt-N3)(2)](2+) (Cyt = cytosine) has been carried out and compared with available condensed phase and X-ray experimental data. Special attention has been paid to relative stabilities of the species with head-tail (ht) and head-head (hh) orientations of the bases and to the interconversion path between them. Rotation of an N3 platinated cytosine nucleobase about Pt-N3 leads to a dramatic lengthening of the Pt-N3 bond as the base reaches coplanarity with the Pt coordination plane, and the ammonia ligands are bent away from the coplanar base. In addition, an unexpected crystal structure of trans-[Pt- (MeNH2)(2)(1-MeC-N3)(2)][PF6](2).H2O (1-MeC = 1-methylcytosine) showing highly twisted nucleobase rings and unusual Pt-N distances has been analyzed. The calculations confirm that this particular crystal structure traps an intermediate of the hh to lit transition. The calculations reveal that the anomalous Pt-N bond length distribution in this crystal structure can be achieved with a minimal energy penalty of less than 2 kcal/mol. To obtain further insights into the platinum-nucleobase interactions three additional complexes have been characterized: trans-[Pt(OH2)(2)(Cyt-N3)(2)](2+), trans-[Pt(OH)(2)(Cyt-N3)(2)], and trans-[PtCl2(Cyt-N3)(2)]. All calculations have been carried out at the DFT (density functional theory) level of theory. Present study demonstrates that contemporary quantum-chemical calculations capturing the intrinsic gas-phase properties of studied species provide a very useful complement to condensed phase and X-ray studies of nucleobase metalation.