The new compound [Co(η5-C5H5)(dppf-P,P′)I]I, 1, was synthesised by the stoichiometric reaction of the Co(III) complex [Co(η5-C5H5)(CO)I2], 2, with 1,1′-bis(diphenylphosphino)ferrocene (dppf) in CH2Cl2, and was characterised by multinuclear NMR spectroscopy. Exposure to air of THF or CH2Cl2 solutions of compound 1 gave, in an unexpected way, a polymeric chain comprising bridging 1,1′-bis(oxodiphenylphosphoranyl)ferrocene (dppfO2) joining tetrahedral Co(II) units [CoI2(μ-dppfO2)]n, 3. Attempts to obtain the polymeric chain 3 by the direct reaction of dppfO2 with CoI2, in CH2Cl2, gave instead the monomeric compound [CoI2(dppfO2)], 4, in which dppfO2 is coordinated in a chelating mode. The structural characterisation of compounds 2, 3, and 4 was carried out by single crystal X-ray diffraction studies. The magnetic behaviour of [CoI2(dppfO2)] and [CoI2(μ-dppfO2)]n was studied, and the results are consistent with tetrahedral S = 3/2 CoII, possessing a 4A2 ground state, and S = 0 FeII. In these compounds, CoII negative zero field splittings were determined from an analysis of the magnetic susceptibility temperature dependence, with D/k = -13 and -14 K for CoI2(dppfO2) and [CoI2(μ-dppfO2)]n, respectively. DFT calculations were performed in order to understand the electronic structure of [Co(η5-C5H5)(dppf-P,P′)I]I, 1, as well as that of the paramagnetic specie [CoI2(dppfO2)], 4. The [CoI2(μ-dppfO2)]n chain was also analysed and found to behave very similarly to the monomeric iodine derivative 4. The calculations showed the unpaired electrons to be localized on the Co(II) centre in all these species. The rather complicated electrochemical behaviour exhibited by the dppf complex [CoIII(η5-C5H5)(dppf-P,P′) I]I and by [Co(dppfO2)I2] is discussed.