The syntheses of the carbonyl substituted clusters Cp’2Mo2Co2S3(CO)4-nLn, n = 1, 2, Cp’ = eta5-C5H4Me, L = PPh3, P(n-Bu)3, P(OMe)3, PPh2H, and PPhH2, are described. Kinetic studies were performed that show the CO substitution involves two elementary steps; an adduct is formed in a preequilibrium step, followed by CO dissociation from the adducts. Equilibrium constants and CO-dissociation rate constants were obtained. The formation of the initial adduct is associative. Typical activation parameters are DELTAH(double dagger) = 24 +/- 3 kcal/mol, DELTAS(double dagger) = 0 +/- 8 eu for CO dissociation and DELTAH(double dagger) = 6 kcal/mol, DELTAS(double dagger) = -28 eu for associative adduct formation with PPh3. Substitution of CO by PPh3 on the cubane cluster Cp2Mo2Co2S4(CO)2 is also associative. DELTAH(double dagger) = 11.3 +/- 0.8 kcal/mol, DELTAS(double dagger) = -24 +/- 3 eu. With Me3P, the adduct Cp’2Mo2Co2S3(CO)4(PMe3) was isolated and its crystal structure determined by X-ray crystallography : monoclinic, space group = Pn (no, 7), a = 7.089(1) angstrom, b = 18.669(6) angstrom, c = 18.469(4) angstrom, beta = 90.84(2)-degrees, Z = 4, d = 2.04 g cm-3, R = 0.040, R(w) = 0.050, GOF = 1.42 based on 4739 reflections with F greater-than-or-equal-to 5sigma(F) and 533 refined parameters. The molecule consists of a slightly folded Co2Mo2 butterfly with a Mo-Mo bond at the hinge (hinge angle = 148-degrees). The Mo2Co(CO)2 triangle has two mu3-bridging sulfide ligands, while the Mo2Co(CO)2(PMe3) triangle has one mu3-S. Some bond distances are Mo-Mo, 2.594(2) angstrom; Mo-Co, 2.692-2.813 angstrom; Mo-S, 2.38 angstrom; Co-S, 2.16 angstrom.