The influence of ligand structure on Ni(III)/Ni(II) redox potentials has been examined in order to elucidate those factors that lead to unusually low values for this couple, such as are found in [NiFe]-hydrogenases. The compound (Et4N)2[Ni(pdtc)2] [3, pdtc = pyridine-2,6-bis(monothiocarboxylate)(2-)] is readily prepared and crystallizes in monoclinic space group C2/c with a = 19.036 (4) Å, b= 11.231 (2) Å, c = 16.588 (3) Å, β = 101.01 (1)°, and Z = 4. Reaction of 3 with iodine in methanol and addition of (Ph3PCH2Ph)Br affords green (Ph3PCH2Ph)[Ni(pdtc)2] (4), which crystallizes in monoclinic space group P21/n with a = 10.181 (1) Å, b = 21.033 (4) Å, c = 16.937 (3) A, β = 95.02 (1)°. and Z = 4. Also prepared was (Ph3PCH2Ph) [Co(pdtc)2], which is isomorphous with 4. Complexes 3 and 4 and [Co(pdtc)2]1-have tetragonally distorted octahedral structures in which the tridentate ligand binds in a meridional manner such that each species has an essentially planar MS4 fragment. The EPR properties of 4 [g⊥> g‖, gav= 2.103, aav(61Ni) = 22 ± 1 G] and comparative structural features with 3 and [Co(pdtc)2]1-demonstrate the Ni(III) state with a (dz:)1ground state. Thus, the 3/4 redox couple, with E1/2= -0.085 V vs SCE in DMF, is a metal-centered process. [Ni(pdtc)2]2-,1-are the first pair of structurally defined Ni(II, III) complexes with anionic sulfur ligands. The previously reported Ni(II) complex [Ni(dapo)2]2-[5, dapo = pyridine-2,6-bis-(acetyloximate)(2-)] exhibited a three-membered redox series, being oxidized to [Ni(dapo)2]1-(6) and then to the known Ni(IV) complex [Ni(dapo)2] (7) at E1/2= -0.735 and -0.255 V in DMF. The tetragonally distorted octahedral structure established earlier for 7 doubtless applies to 6, which could not be isolated. However, the EPR spectrum of 6, although poorly resolved, has gav= 2.086, indicating that the 5/6 redox couple is metal-centered. The potential of this couple is one of the two lowest values reported for NiIII/II. These results make clear that, in the absence of any pronounced structural stabilization of one oxidation state, the dominant ligand electronic property in lowering redox potentials is the presence of polarizable, electron-rich donor atoms or groups. Low potentials (ca. -400 to -600 mV vs SCE) are a prominent feature of [NiFe]-hydrogenases. Certain properties of these synthetic systems are compared with those of enzymes, and suggestions as to the nature of native Ni sites are offered. © 1990, American Chemical Society. All rights reserved.