The synthesis of four Ni^II formate complexes of the type [Ni(N^∩N)_n][O₂CH]₂ (2, adduct with 3/4 EtOH, N^∩N = en, ethylenediamine, n = 3; 4, N^∩N = dien, N,N′,N′′-diethylenetriamine, n = 2), [Ni₂(O₂CH)₄(H₂O)(tmeda)₂] (3, tmeda = N,N,N′,N′-tetramethylethylenediamine) and [{Ni(O₂CH)₂(pmdta)}₂·H₂O] (5, pmdta = N,N′,N′,N′′,N′′-pentamethyldiethylenetriamine) by a reaction of [{Ni(O₂CH)₂}·2H₂O] (1) with the respective N-donor bases is reported. The structures of 2-5 in the solid state were determined by single X-ray structure analysis, revealing a discrete dinuclear structure of 3 and the formation of polymeric networks in the case of 2, 4 and 5 due to intermolecular hydrogen bonding. SQUID and ESR measurements of 3 evidenced a weak antiferromagnetic coupling between the Ni^II ions and an easy plane magnetic anisotropy. Accompanying quantum chemical studies of the magnetic properties and IR characteristics of 3 were performed to strengthen the conclusions drawn from experimentally obtained data. The thermal decomposition temperatures of 2-5 were determined by TG (thermogravimetry) and obtained residues were analyzed by PXRD (powder X-ray diffraction) measurements. The decomposition processes were completed at 207 (3), 215 (5), 250 (2) and 273 °C (4) and are shown to result in the formation of pure metallic nickel.