A new complex {[Co(hfac)(2)](2)(4P3NIT)(2)}center dot CHCl3 (1) (hfac: hexafluoroacetylacetonate, 4P3NIT: 2-[3-(4-pyridyl)phenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) has been synthesized and characterized structurally and magnetically. The single-crystal structure revealed that two 4P3NIT molecules and two [Co(hfac)(2)] units construct a four-spin rectangle-like molecule. Effective-spin approach was also employed to evaluate the magnetic coupling J(1) and J(2) within this four-spin unit. The best fitting result gives J(1) = -23.41 cm(-1), J(2) = 3.18 cm(-1), which indicate the anti ferromagnetic interaction between the Co(II) ion and directly bonded radical, and weak ferromagnetic interaction between the central Co(II) ion and organic radical through the pathway of pyridyl and phenyl ring.