cis-[PtCl(OH2)(NH3)2]+, the monoactivated form of cisplatin, reacts with the cyano ligand of cobalt in vitamin B12 (cyanocobalamin) to form a Co-C[triple chemical bond]N-Pt conjugate (1). Compound 1 is prepared in good yield directly in aqueous solution. The remaining chloride ligand of Pt(II) is labile. It hydrolyzes slowly in aqueous solution and can be exchanged by stronger coordinating ligands, such as 9-methylguanine or 2'-deoxyguanosine, to yield vitamin B12-nucleobase conjugates. X-ray structures of the vitamin B12-cisplatin conjugate 1 as well as of the product with coordinated 9-methylguanine (2) are presented. The coordination geometry at Pt(II) is almost perfectly square-planar. The structure of the cobalamin compound remains essentially unchanged when compared with the original B(12) structure. The guanine moiety of compound 2 binds in a 45 degrees angle to the cisplatin molecule and interacts with neighboring molecules by means of pi stacking and hydrogen bonds.