Three non-native derivatives of vitamin B12 with imidazole, ethylenediamine, and pyrazine as cobalt(III) β-ligands were characterized by applying the BP/def2-TZVP density functional method. The binding of all three ligands is thermodynamically favorable. It is proposed that their synthesis might be possible from aquacobalamin as a starting form of vitamin B12, as has been done in the case of an imidazole derivative of B12 (Hannibal et al. Inorg. Chem. 2007, 46, 3613-3618). Furthermore, the possibility of the formation of their conjugates with cisplatin is investigated. The proposed β-ligands may serve as bridging ligands, binding to the platin ion as N-donors. In parallel, the calculations are done for the previously synthetized B12-cisplatin adduct with CN(-) as a bridging ligand and are compared with available experimental data, allowing assessment of the applied computational protocol. A good agreement between the computed and experimental structural parameters is obtained. In each of the studied structures, the Co-β-ligand bond is weaker than the Pt-β-ligand bond.