A new planar polynitrile dianion ([tcno]2− = [(NC)2CC(O)C(CN)2]2−) has been synthesized as its potassium salt, K2[tcno] (1). The crystallization of 1 by the slow evaporation of an aqueous solution at room temperature gave two types of colourless crystals having two different shapes [1-A: fine plates and 1-B: needles] for which the crystal structure determinations showed similar geometries for the polynitrile anion in both the structures. The combination of this novel dianion with Cu(II) led to the coordination complex [Cu(tcno)2(H2O)2] (2), which constitutes the first coordination complex of this dianion. The structure of 2 can be described as a coordination polymer in which the organic dianion acts as a μ3-bridging ligand. Despite the fact that the Cu⋯Cu bridges have a large number of atoms, the Cu⋯Cu distance observed through this ligand is relatively short (Cu⋯Cu 3.644 Å) compared to those reported for several polynitrile metal complexes (>6 Å).