Abstract
Isocytosine (ICH; 1) exists in solution in an equilibrium of tautomers 1 a and 1 b with the N^1^ and N^3^ positions carrying the acidic proton, respectively. In the solid state, both tautomers coexist in a 1:1 ratio. As we show, the N^3^H tautomer 1 b can selectively be crystallized in the presence of the model nucleobase 1‐methylcytosine (1‐MeC). The complex 1 b⋅(1‐MeC)⋅2 H~2~O (2) forms pairs through three hydrogen bonds between the components; hydrogen bonds between identical molecules are also formed, leading to an infinite tape structure. On the other hand, the N^1^H tautomer 1 a co‐crystallizes with protonated ICH to give [1 a⋅ICH~2~]NO~3~ (3), again with three hydrogen bonds between the partners, yet the acidic proton is disordered over the two entities. With M^II^(dien) (M=Pt, Pd; dien=diethylenetriamine) preferential coordination of tautomer 1 a through the N3 position is observed. DFT calculations, which were also extended to Pt^II^(tmeda) linkage isomers (tmeda=N,N,N′,N′‐tetramethylethylenediamine), suggest that intramolecular hydrogen bonding between the ICH tautomers and the co‐ligands at M, while adding to the preference for N^3^ coordination, is not the major determining factor. Rather it is the inherently stronger Pt__N__^3^ bond which favors complexation of 1 a. With an excess of M^II^(dien), dinuclear species [M~2~(dien)~2~(IC‐N^1^,N^3^)]^3+^ (M=Pd^II^, 4 and Pt^II^, 5) also form and were isolated as their ClO~4~^−^ salts and structurally characterized. In strongly acidic medium 5 is converted to [Pt(dien)(ICH‐N^1^)]^2+^ (6), that is, to the Pt^II^ complex of tautomer 1 b.