The conversion of P4 with [CpBnCo(CO)2] (CpBn=C5(CH2C6H5)5) under thermal conditions gives almost exclusively the P10 complex [(CpBnCo)4(μ4-η4:η4:η2:η2:η1:η1-P10)]. However, its reaction with [CpBnCo]n, under mild conditions afforded a mixture of products including the P8 species [(CpBnCo)3(μ3-η4:η2:η2:η1:η1-P8)] and a rare Co3 cluster [(CpBnCo)2Co(μ3-η5:η2:η1:η1-C40H33)(μ-H)(μ3-H)]. The products were studied in solution by multinuclear NMR spectroscopy and mass spectrometry and, for some compounds in the solid state, by single crystal X-ray diffraction.