The syntheses, structures and redox properties of isomorphous Ni(II) thiolato and selenolato complexes of the tridentate ligands bis(2-(hydrochalcogeno)ethyl)methylamine are reported. Reaction of Ni(OAc)2 with bis(2-mercaptoethyl)methylamine leads to the formation of a dimeric complex, bis{[(mu-2-mercaptoethyl)(2-mercaptoethyl)methylaminato (2-)]nickel(II)}, [Ni(1)](2). This complex contains planar, diamagnetic Ni(II) centers ligated by a tertiary amine N-donor atom, a terminal thiolate, and two thiolates that bridge,to the second Ni center in the dimer. Crystals of [Ni(1)](2) form in orthorhombic space group Pna2(1) with cell dimensions a 19.695(2) Angstrom, b 6.042(2) Angstrom, c 13.463(3) Angstrom, V = 1602(1) Angstrom(3), and Z = 4. Reaction-of Ni(OAc)2 with bis(2-(hydroseleno)ethyl)methylamine results in the formation of a structurally analogous dimeric complex, bis{[(mu-2-(hydro- seleno)ethyl)(2-(hydroseleno)ethyl)methylaminato(2-)]nickel(II)}, [Ni(2)](2), where all of the chalcogenolate donors are selenolates. Crystals of [Ni(2)](2) are isomorphous with those of [Ni(1)](2), with a = 20.040(8) Angstrom, b = 6.265(2) Angstrom, c = 13.590(5) Angstrom, and V = 1706(2) Angstrom(3). One-electron oxidation of either dimeric complex leads to the formation of radical cations, which exhibit EPR spectra consistent with S = 1/2 radicals. For [Ni(1)](2+) the g values observed (g(x) = 2.20, g(y) = 2.14; g(z) = 2.02) are essentially identical to those observed for a reduced and catalytically viable redox state of Fe,Ni hydrogenases (g(x) = 2.20, g(y) = 2.14, g(z) = 2.05). The substitution of Se- for donors in [Ni(2)](2) does not alter the Observed g values much (g(x) =- 2.23, g(y) = 2.14, g(z) = 2.05) but leads to the observation of Se-77 hyperfine coupling (A(z) = 129 G) that indicates that the molecular orbital containing the unpaired spin is largely Se in character (54%). Reaction of either dimeric complex with CN- leads to the formation of mononuclear trans-dichalcogenolate complexes, [Ni(1)CN](-) and [Ni(2)CN](-). Exposure of [Ni(1)CN](-) to O-2 leads to the quantitative formation of a monosulfinato complex. In contrast, the selenolato complex does not react with O-2 under the same conditions. The role of selenocysteinate ligation in Fe,Ni,Se hydrogenases is discussed in view of this chemistry.