AbstractThe hydrothermal reaction of 2,3‐pyridinedicarboxylic acid (2,3‐H2pda) with a mixture of Cd(NO3)2and Ni(NO3)2afforded a coordination polymer, [CdNi(2,3‐pda)2(H2O)3]∞(1); in contrast, that with a mixture of Cd(NO3)2and Zn(NO3)2surprisingly produced a discrete molecule, trans‐[Cd(3‐pa)2(H2O)4] (2) (3‐pa−= 3‐pyridinecarboxylate). Since a direct reaction between a single metal salt, Cd(NO3)2or Zn(NO3)2, and 3‐pyridinecarboxylic acid (3‐Hpa) under similar hydrothermal conditions yielded different coordination polymers containing 3‐pa−, it appears that the apparently thermal decarboxylation from ligated 2,3‐pda2−to 3‐pa−occurs after complexation of both metal cations, Cd(II) and Zn(II). A new coordination mode, formed for 2,3‐pda2−in structure1, appears to help formation of microporous channels by piling up the observed 2D hydrogen‐bonded heteropolynuclear layers. Each channel apparently consists of two in