Thermolysis of the tetranuclear cobalt cluster [Co 4 (μ 3 -AsPh)(μ 4 -η 2 :η 2 :η 1 -As 4 Ph 4 )(μ-CO) 2 (CO) 8 ] (1) at 60 o C in benzene for two days gives [Co 4 (μ 3 -AsPh) 2 (μ 3 -η 2 :η 1 -As 2 Ph 2 )(μ-CO)(CO) 9 ] (2), in which the tetraarsine chain in 1 has been cleaved to give the bridging As 2 Ph 2 unit in 2. Similar fragmentation of the As 4 Ph 4 chain occurs on treatment of 1 with [Fe 2 (CO) 9 ] in benzene at room temperature for 3 days affording [Co 4 (μ 3 -AsPh) 2 {(μ 3 -η 2 :η 1 -As 2 Ph 2 )Fe(CO) 4 }(μ-CO)(CO) 9 ] (3), in which one arsine atom of the As 2 Ph 2 bridging ligand is additionally bound to an Fe(CO) 4 unit. Conversely, reaction of 1 with excess P(OMe) 3 at room temperature in benzene for 3 days gives the trisubstituted trimethylphosphite derivative [Co 4 (μ 3 -AsPh)(μ 4 -η 2 :η 2 :η 1 -As 4 Ph 4 )(μ-CO) 2 (CO) 5 {P(OMe) 3 } 3 ] (4), in which the As 4 Ph 4 unit remains intact. Single crystal X-ray diffraction studies have been performed on 2, 3 and 4 and possible reaction pathways for the formation of the new complexes are proposed and discussed.