Copper(II) formate is an inorganic salt derived from formic acid. It exists in three crystalline modifications – anhydrous salt, dihydrate and tetrahydrate. The hydrates do not retain their structures upon dehydration . All three modifications adopts three-dimensional structures where formate ion acts as a bridging ligand.
- Anhydrous copper(II) formate is prepared by a reaction of concentrated formic acid (90%) in excess with copper carbonate at elevated temperatures (ca. 100 °C). The product is separated in a form of well-formed royal-blue crystals. It was structurally characterized by Barclay and Kennard in 1961 . It crystallizes in orthorombic crystal system and the structure is described as three-dimensional array of copper(II) atoms joined together by formate ions in an anti-syn arrangement.
- Copper(II) formate dihydrate is prepared by a reaction of concentrated formic acid (90%) in excess with copper carbonate followed by treatment with a large excess of diethylether. The product is separated in a powder form . The salt crystallizes in monoclinic crystal system. The structurewas determined by Bukowska-Strzyewska in 1964 . In the structure anti-syn and anti-anti arrangement of bridging formate ions is observed. The salt is said to be metastable.
- Copper(II) formate tetrahydrate is prepared by a reaction of diluted formic acid (20%) in excess with copper carbonate. The product is then subsequently separated in a form of blue crystals. In the structure, anti-anti bridging formate ions are found. The solubility of the salt in water at 20 °C is 125 g/L.
A solubility polytherm for Cu(formate)2/water system shows that in a temperature interval 4-70 °C the solid form of product is dependent on the mass percentage of Cu(formate)2 in the binary system . The tetrahydrate thus crystallizes only at temperatures below ca. 42 °C and the anhydrous copper(II) formate crystallizes above 60 °C. In other articles, differencies from this polytherm are found. An additional work should be brought up to determine solubility more precisely.
A suspension of copper(II) carbonate (5 g, 40.5 mmol) was made by adding the compound into 25 mL of distilled water. Under constant stirring formic acid (3.6 mL, 81 mmol) was added to the suspension. The mixture was stirred until all solid disappeared leaving dark blue solution of copper(II) formate. The solution was let freely to crystallize. Yield: 80,9 %.
 R. L. Martin and H. Waterman, J. Chem. Soc., 1359 (1959).
 G. A. Barclay arid and C. H. L. Kennard, J. Chem. Soc., 3289 (1961).
 A. K. Galwey, D. Jamieson, and M. E. Brown, J. Phys. Chem. A, 2664 (1974).
 M. Bukowska-Strzyzewaka, Acta Crystallogr., 19, 357 (1965).
 N. Y. Ostanni et al., Zh. Fiz. Khim. 48, 2583 (1974).