The reactions of [Co₂(µ-As₂)(CO)₆] with [Mo₂Cp₂(CO)₄] (Cp = η⁵-C₅H₅) and with [(OC)₄CoMCp(CO)₃] [M = Mo or W] gave, respectively, the known product [Mo₂(µ-As₂)Cp₂(CO)₄] 1 and the new heterobimetallic complexes [(OC)₃Co(µ-As₂)MCp(CO)₂] [M = Mo 2 or W 3]. The mixed-metal complexes have been treated with PPh₃ to give the substituted complexes [(Ph₃P)(OC)₂Co(µ-As₂)MCp(CO)₂] [M = Mo 4 or W 5]. Complex 2 reacted with [Cr(CO)₅]·THF to give [(OC)₃Co{µ-As₂[Cr(CO)₅]₂}MoCp(CO)₂] 6. Reaction of [MoWCp₂(CO)₄] with [Co₂(µ-As₂)(CO)₆] does not lead to the complex [MoW(µ-As₂)Cp₂(CO)₄] although this can be obtained by co-thermolysis of [Mo₂Cp₂(CO)₆] and [W₂Cp₂(CO)₆] with grey arsenic which affords three bimetallic products, [MM′(µ-As₂)Cp₂(CO)₄] (M = M′ = Mo 1; M = Mo, M′ = W 7; M = M′ = W), and four trimetallic products, [M₂M′(µ₃-As)Cp₃(CO)₆] (M = M′ = Mo; M = Mo, M′ = W 8; M = W, M′ = Mo 9; M = M′ = W). The new heterometallic complexes 2, 3, 5, 6 and 9 have been characterized by X-ray diffraction analyses.