Treatment of the dibridged dicobalt complex [Co₂(μ-PPh₂){μ-η¹(S)∶η¹(C)∶η²(C)-SPhC(O)CR′CR}(CO)₄] (R′ = R = H 1a) with L, at ambient temperature, affords an inseparable mixture of two mono-substituted isomeric complexes of general formula [Co₂(μ-PPh₂){μ-η¹(S)∶η¹(C)∶η²(C)-SPhC(O)CHCH}(CO)₃(L)] [L = P(OMe)₃2a/3a, PPhMe₂2b/3b], differing only in the metal centre at which L is coordinated. When the reaction is performed with [Co₂(μ-PPh₂){μ-η¹(S)∶η¹(C)∶η²(C)-SPhC(O)CR′CR}(CO)₄] [R′ = Me, R = H 1b], in which R′ and R are inequivalent, a similar mixture of inseparable isomers is isolated, [Co₂(μ-PPh₂){μ-η¹(S)∶η¹(C)∶η²(C)-SPhC(O)CMeCH}(CO)₃(L)] [L = P(OMe)₃4a/5a, PPhMe₂4b/5b] and, in addition, a third separable isomer, [Co₂(μ-PPh₂){μ-η¹(S)∶η¹(C)∶η²(C)-SPhC(O)CHCMe}(CO)₃(L)] [L = P(OMe)₃, 6a, PPhMe₂6b], in which R′ and R have interchanged. Addition of a further equivalent of L at elevated temperature to mixtures of 2/3 results in both carbonyl substitution and a rearrangement of the bridging ligand assembly to give [Co₂(μ-SPh){μ-η¹(P)∶η¹(C)∶ η²(C)-PPh₂CRCR′C(O)}(CO)₂(L)₂] [R′ = R = H; L = P(OMe)₃7a, PPhMe₂7b] in high yield. A similar transformation is achieved on interaction of 4/5 and 6 with L leading to [Co₂(μ-SPh){μ-η¹(P)∶η¹(C)∶η²(C)-PPh₂CRCR′C(O)}(CO)₂(L)₂] [R′ = Me, R = H; L = P(OMe)₃8a, PPhMe₂8b] and, in addition, [Co₂(μ-SPh){μ-η¹(P)∶η¹(C)∶η²(C)-PPh₂CR′CRC(O)}(CO)₂(L)₂] [R′ = Me, R = H; L = P(OMe)₃9a, PPhMe₂, 9b], in which the positions of R′ and R are reversed. Reaction of 2a/3a with triphenylphosphine, gives the mixed phosphite–phosphine rearranged species [Co₂(μ-SPh){μ-η¹(P)∶η¹(C)∶η²(C)-PPh₂CHCHC(O)}(CO)₂{P(OMe)₃}(PPh₃)] 10 as the sole product. The facility of the bridging ligands in 1 to undergo a range of bond-breaking (C–S and C–C) and bond-forming (C–S, C–P and C–C) reactions induced by phosphorus donor ligand addition is discussed and pathways postulated. The single crystal X-ray structures of 2b, 7a and 10 are described.