Aliphatic thiolato-S-bridged tri- and binuclear nickel(II) complexes have been synthesized and characterized as models for the Ni-p site of the A cluster of acetyl coenzyme A synthase (ACS)/carbon monooxide.(CO) dehydrogenase. Reaction of the in situ formed N2Sthiol donor ligands with [Ni(H2O)(6)](ClO4)(2) afforded the trinuclear complexes [Ni-(L-Me(S))(2)Ni}(2)](ClO4)(2)center dot CH3CN (1 center dot CH3CN) and [Ni-{(L-Br(S))(2)Ni}(2)](CIO4)(2)center dot 5H(2)O (2 center dot 5H(2)O) following self-assembly. Complexes 1 and 2 react with [Ni(dppe)Cl-2] and dppe [dppe = 1,2-bis(diphenylphosphino)ethane] to afford the binuclear [Ni(dppe)Ni(L-Me(s))(2)](ClO4)(2)center dot 2H(2)O (3 center dot 2H(2)O) and [Ni(dppe)Ni(L-Br(s))(2)](ClO4)(2)center dot 0.750(C2H5)(2) [4.0.750-(C2H5)(2)], respectively. The X-ray crystal structures of 1-4 revealed a central (NiS4)-S-II moiety in 1 and 2 and a (NiP2S2)-P-II moiety in 3 and 4; both moieties have a square-planar environment around Ni and may mimic the properties of the Nip site of ACS. The electrochemical reduction of both terminal Ni-II ions of 1 and 2 occurs simultaneously, which is further confirmed by the isolation of [Ni{(L-Me(s))(2)Ni(NO)}(2)](ClO4)(2) (5) and [Ni{(L-Br(s))(2)Ni(NO)}(2)](ClO4)(2) (6) following reductive nitrosylation of 1 and 2. Complexes 5 and 6 exhibit nu(NO) at 1773 and 1789 cm(-1), respectively. In the presence of 02, both 5 and 6 transform to nitrite-bound monomers [(LMe(S-S))Ni(NO2)]-(ClO4) (7) and [LBr(S-S))Ni(NO2)](ClO4)(2) (8). The nature of the ligand modification is evident from the X-ray crystal structure of 7. To understand the origin of multiple reductive responses of 1-4, complex [(L-Me(SMe))(2)Ni](ClO4)(2) (9) is considered. The central NiS4 part of 1 is labile like the Nip site of ACS and can be replaced by phenanthroline. The treatment of CO to reduce 3 generates a 3(red)-(CO)(2) species, as confirmed by Fourier transform infrared (nu(CO) = 1997 and 2068 cm(-1)) and electron paramagnetic resonance (g(1) = 2.18, g(2) = 2.13, g(3) = 1.95, and A(P) = 30-80 G) spectroscopy. The CO binding to Ni-I of 3(red) is relevant to the ACS activity.