New dodecanuclear phenylphosphonate-bridged Co(II) complexes [(μ 2 -THF) 5 (η-HPiv)Co 12 (μ 3 -OH) 4 (μ 6 -O 3 PPh) 4 (μ-Piv) 12 ]·THF·2С 6 Н 6 (1·THF·2С 6 Н 6 ), [(μ 2 -THF) 6 Co 12 (μ 3 -OH) 4 (μ 6 -O 3 PPh) 4 (μ-Piv) 12 ]·3THF (2·3THF) and [(μ 2 -Hmhp) 6 Co 12 (μ 3 -OH) 4 (μ 6 -O 3 PPh) 4 (μ-Piv) 12 ]·6.5MeCN (3·6.5MeCN) have been synthesized by the reaction of the cobalt(II) pivalate polymer {Co(Piv) 2 } n (Piv is the pivalate anion), as a source of metal fragments containing cobalt(II) atoms, with potassium phenylphosphonate (K 2 PO 3 Ph) in the presence of neutral O-donor additional ligands like molecules of THF or 6-methyl-2-pyridone (Hmhp). The structures of the dodecanuclear complexes 1–3 were determined by using single-crystal X-ray diffraction data. The magnetic properties of the compounds showed that complexes 1 and 3 exhibit antiferromagnetic exchange interactions between the magnetic centers. The stability of compounds 1 and 3 were studied using thermogravimetric analysis.