The new heterobimetallic complexes of general formula [{(R′S)Ph₂P}(OC)₂Co{µ-C₂(CO₂Me)₂}M(η⁵-L)(CO)₂] 1 [L = C₅H₅ or C₅Me₅; M = Mo or W; R′ = hydrocarbyl] undergo thermally induced phosphorus–sulfur bond breaking reactions. By variation of the nature of the R′ substituent, the cyclopentadienyl group or the Group 6 metal centre in 1, bimetallic complexes incorporating bridging four-membered metallacycles of the type M–P–CC or M–S–CC (where M = Co, Mo or W), are accessible. While [{(PhS)Ph₂P}(OC)₂Co{µ-C₂(CO₂Me)₂}Mo(η⁵-C₅H₅)(CO)₂] 1a gives as the sole product the phosphorus-containing molybdenacyclic species [(OC)(η⁵-C₅H₅)Mo{µ-PPh₂C(CO₂Me)C(CO₂Me}(µ-SPh)Co(CO)₂] 2a, the corresponding reaction of [{(R′S)Ph₂P}(OC)₂Co{µ-C₂(CO₂Me)₂}Mo(η⁵-C₅H₅)(CO)₂] (R′ = Buⁿ 1b; Bu^t 1c), results in the formation of both [(OC)(η⁵-C₅H₅)Mo{µ-PPh₂C(CO₂Me)C(CO₂Me)}(µ-SR′)Co(CO)₂] (R′ = Buⁿ 2b; Bu^t 2c) and [(OC)₂Co{µ-SR′C(CO₂Me)C(CO₂Me)}(µ-PPh₂)Mo(η⁵-C₅H₅)(CO)] (R′ = Buⁿ 3b; Bu^t 3c). The latter sulfur-containing cobaltacyclic species (3c) has been shown to isomerise to the phosphorus-containing molybdenacyclic species 2c on further heating. Conversely, employment of the pentamethylcyclopentadienyl complex [{(PhS)Ph₂P}(OC)₂Co{µ-C₂(CO₂Me)₂}Mo(η⁵-C₅Me₅)(CO)₂] 1b gives three species, [(OC)(η⁵-C₅Me₅)Mo{µ-PPh₂C(CO₂Me)C(CO₂Me)}(µ-SPh)Co(CO)₂] 2d and [(OC)₂Co{µ-SPhC(CO₂Me)C(CO₂Me)}(µ-PPh₂)Mo(η⁵-C₅Me₅)(CO)] 3d, which are, respectively, analogues of 2a–3c and 3b, 3c, and in addition the phosphorus-containing cobaltacyclic species [(OC)₂Co{µ-PPh₂C(CO₂Me)C(CO₂Me)}(µ-SPh)Mo(η⁵-C₅Me₅)(CO)] 4d. Thermolysis of [{(BuⁿS)Ph₂P}(OC)₂Co{µ-C₂(CO₂Me)₂}W(η⁵-C₅H₅)(CO)₂] 1e, in which a tungsten centre has been introduced in place of the molybdenum in 1b, affords only sulfur-containing metallacyclic products, which incorporate either the cobalt centre as in [(OC)₂Co{µ-SBuⁿC(CO₂Me)C(CO₂Me)}(µ-PPh₂)W(η⁵-C₅H₅)(CO)] 3e or the tungsten centre as in [(OC)(η⁵-C₅H₅)W{µ-SBuⁿC(CO₂Me)C(CO₂Me)}(µ-PPh₂)Co(CO)₂] 5e. The ‘flyover’ complex [(OC)(η⁵-C₅H₅)W{µ-C(CO₂Me)CHC(OMe)O}(µ-SPh)Co(CO){PPh₂(SPh)}] 6 was the only product obtained from the reaction of [(OC)₃Co{µ-C₂(CO₂Me)₂}W(η⁵-C₅H₅)(CO)₂] with Ph₂P(SPh). Single crystal X-ray diffraction studies have been performed on complexes 1d, 2b, 3c′ [the P(OMe)₃-substituted derivative of 3c], 5e and 6.