Current mechanistic proposals for the acetyl synthase activity of nickel-containing carbon monoxide dehydrogenases (CH3-THF + CoA . SH --> CoA . SCOCH3 + THF; THF = tetrahydrofolate, CoA . SH = coenzyme A) implicate a Ni catalytic center and the steps [Ni-CH3] --> [Ni-COCH3] --> CoA . SCOCH3. The second step presumably involves attack by the sulfur nucleophile of coenzyme A at the acyl carbon atom in the overall reaction [Ni-II-COCH3] + RS(H) --> RSCOCH(3) + Ni-II (+ H+) + 2e(-). We have previously demonstrated these steps in Ni(II) complexes with physiological-type ligation. In this work, it is shown that the reaction of acyl and thiolate ligands coordinated to Ni(II) affords thioesters in high yield, The complex [Ni(bpy)(CH3)(2)], established to be planar by an X-ray structure determination, reacts with 1 equiv of arenethiol to afford diamagnetic planar [Ni(bpy)(CH3)(SR)] (8) with R = p-C6H4CH3, 2,6-C6H3(CH3)2 (8b), mesityl (8c), 2,4,6-C(6)H(2)iPr(3), and 2,6-C6H3Cl2 (8e) (bpy = 2,2’-bipyridyl). An analogous reaction gives [Ni(bpy)(C2H5)(S-mesityl)] (12) from [Ni(bpy)(C2H5)(2)]. Planar structures of 8c,e were confirmed by X-ray analysis. Complexes 12 and 8 with different R substituents undergo thiolate ligand exchange in THF with K-eq approximate to 1. Reaction of 8e with 1 equiv of carbon monoxide yields the acyl complex [Ni(bpy)(COCH3)(S-2,6-C6H3Cl2)] (9a), whose planar coordination unit was confirmed by X-ray methods. Treatment of the complexes 8 in THF with more than 3 equiv of carbon monoxide yields [Ni(bpy)(CO)(2)] and the thioesters RSCOCH(3) in 96-100% yield in situ. A solution initially containing 8b and 12 gave under the same conditions four thioesters in equal amounts, consistent with four complexes in the equilibrated solution prior to reaction with carbon monoxide. Reaction of 9a in THF with carbon monoxide produced 2,6-dichlorophenyl thioacetate quantitatively, indicating that Ni(II)-acyl-thiolate complexes are intermediates in thioester formation. The overall reaction is [Ni(bpy)(R’)(SR)] + 3CO --> RSCOR’ + [Ni(bpy)(CO)2] (R’ = CH3, C2H5); the two electrons in the generalized reaction are captured by the metal as Ni(0). A related and necessarily intramolecular reaction of [Ni(bpy)(SCH2CH2CH2)] was confirmed and shown to produce gamma-thiobutyrolactone in quantitative yield in situ. Evidence supporting an analogous intramolecular path for reaction systems based on 8 is summarized. This investigation provides the first examples of Ni-mediated acyclic thioester synthesis and demonstrates a possible means of enzymatic thioester formation should coenzyme A(-) and an acetyl group coordinate to the Ni(II) catalytic center.