The complex [(OC)₃Co{µ-C₂(CO₂Me)₂}Mo(CO)₂(η⁵-C₅H₅)]1 reacts preferentially at the Co atom with PPh₂H, P₂Ph₄ and PPh₂(SPh) to give initially the mono-substituted products [L(OC)₂Co{µ-C₂(CO₂Me)₂}Mo(CO)₂(η⁵-C₅H₅)][L = PPh₂H 2, P₂Ph₄3 or PPh₂(SPh)4]. In the case of PPh₂H further reaction takes place at the Mo atom to give the bis-substituted product [(HPh₂P)(OC)₂Co{µ-C₂(CO₂Me)₂}Mo(CO)(η⁵-C₅H₅)(PPh₂H)]5. Thermolysis of the substituted derivatives 2–5 results in either P–H, P–P or P–S bond cleavage. Thus for 2, the isomeric species [(η⁵-C₅H₅)(OC)Mo{µ-C(CO₂Me)CH(CO₂Me)}(µ-PPh₂)Co(CO)₂]6 and [(OC)₂Co{µ-C(CO₂Me)CH(CO₂Me)}(µ-PPh₂)Mo(CO)(η⁵-C₅H₅)]7 are obtained, whereas 3 gives [(OC)₂Co{µ-PPh₂C(CO₂Me)C(CO₂Me)}(µ-PPh₂)Mo(CO)(η⁵-C₅H₅)]8, in which the Co atom is incorporated into a four-membered metallacycle. Thermolysis of 4 gives [(η⁵-C₅H₅)(OC)Mo{µ-PPh₂C(CO₂Me)C(CO₂Me)}(µ-SPh)Co(CO)₂]9(in which the Mo atom is incorporated into a four-membered metallacycle) and thermolysis of 5 gives [(η⁵-C₅H₅)(OC)Mo{µ-C(CO₂Me)CH(CO₂Me)}(µ-PPh₂)Co(CO)(PPh₂H)]10. Complex 6 can be converted to 10 by reaction with PPh₂H. The crystal structures of compounds 6 and 8 have been determined.