Flat vs. Folded Chelate Rings in cis‐ Pt II a 2 (a = NH 3 , a 2 = en, a 2 = 2, 2′‐bpy) Complexes of Twofold Substituted Diazine Ligands

Acid‐base and ligating properties of three bis(substituted)pyrazine (pz) and pyrimidine (pym) ligands (pyrazine‐2, 5‐dicarboxylic acid, 2, 5‐pzdcH2, 2, 3‐bis(pyridine‐2‐yl)pyrazine, 2, 3‐bppz, pyrimidine‐4, 6‐dicarboxylic acid, 4, 6‐pmdcH 2 ) toward cis ‐Pt II a 2 (a = NH 3 , a 2 = en, a 2 = 2, 2′‐bpy) have been studied. Combinations of pz‐N/pym‐N with donor atoms of the substituents lead to 5‐membered platinum chelates, but exclusive N, N‐coordination through the pyridyl substituents of 2, 3‐bppz can lead to a 7‐membered platinum chelate with a characteristic L‐shape of the resulting cation. It is observed for Pt II (2, 2′‐bpy), yet not for Pt II (en), and is a consequence of differences in sterical interactions between the 2, 3‐bppz ligand and the coligands of Pt II .