The compound [{Cu(tzq) ₂ (HCO ₂ )} ₂ (µ-HCO ₂ ) ₂ ]·4H ₂ O (tzq = [1,2,3]triazolo[1,5-a]quinoline) was obtained under mild conditions by reaction between the hydrazone of quinoline-2-carbaldehyde and copper(II) formate in ethanolic aqueous solution and its crystal and molecular structure have been determined by X-ray diffraction methods: triclinic, space group P, Z = 1, a = 8.209(4), b = 12.156(4), c = 12.649(4) Å, α = 63.21(3), β = 81.42(3), γ = 89.12(3)°. The compound is built of centrosymmetric neutral dimeric [{Cu(tzq) ₂ (HCO ₂ )} ₂ (µ-HCO ₂ ) ₂ ] entities linked through hydrogen bonds involving water molecules and carboxylate groups. The copper atom is involved in a CuN ₂ O ₂ O′ chromophore and lies in a tetrahedrally distorted square-pyramidal environment. The four equatorial donors are two trans nitrogen atoms from two tzq molecules and two oxygen atoms from two formate groups. One of the oxygens acts as a bridging ligand occupying the apical position of the symmetry-related copper atom in the dimer structure, which is viewed as two edge-sharing distorted square pyramids. This co-ordination behaviour (monoatomic bridging) is very unusual for the HCO ₂ ^- anion. The EPR spectrum is characteristic of a triplet state with non-negligible zero-field splitting. Magnetic susceptibility measurements in the range 1.8–200 K showed weak antiferromagnetic exchange between the copper(II) ions (2J = -1.05 cm ^-1 ). The observed behaviour is discussed on the basis of the crystal structure and compared with those reported for related µ-oxo carboxylate-bridged [CuO ₂ Cu] dimers with parallel-planar geometry.