A series of five-coordinate thiolate-ligated complexes [M II (tren)N 4 S Me2 ] + (M = Mn, Fe, Co, Ni, Cu, Zn; tren = tris(2-aminoethyl)amine) are reported, and their structural, electronic, and magnetic properties are compared. Isolation of dimeric [Ni II (SN 4 (tren)-RS dang )] 2 ("dang"= dangling, uncoordinated thiolate supported by H-bonds) using the less bulky [(tren)N 4 S] 1− ligand, pointed to the need for gem-dimethyls adjacent to the sulfur in order to sterically prevent dimerization. All of the gem-dimethyl derivatized complexes are monomeric, and with the exception of [Ni II (S Me2 N 4 (tren)] + , are isostructural and adopt a tetragonally distorted trigonal bipyramidal geometry favored by ligand constraints. The nickel complex uniquely adopts an approximately ideal square pyramidal geometry, and resembles the active site of Ni-superoxide dismutase (Ni-SOD). Even in coordinating solvents such as MeCN, only five-coordinate structures are observed. The M II -S thiolate bonds systematically decrease in length across the series (Mn-S > Fe-S > Co-S > Ni-S ~ Cu-S < Zn-S) with exceptions occurring upon the occupation of σ* orbitals. The copper complex, [Cu II (S Me2 N 4 (tren)] + , represents a rare example of a stable Cu II -thiolate, and models the perturbed "green" copper site of nitrite reductase. In contrast to the intensely colored, low-spin Fe (III)-thiolates, the M(II)-thiolates described herein are colorless to moderately colored, and highspin (in cases where more than one spin-state is possible), reflecting the poorer energy match between the metal d-and sulfur-orbitals upon reduction of the metal ion. As the d-orbitals drop in energy proceeding across the across the series M 2+ (M= Mn, Fe, Co, Ni, Cu), the sulfur-to-metal charge transfer transition moves into the visible region, and the redox potentials cathodically shift. The reduced M +1 oxidation state is only accessible with copper, and the more oxidized M +4 oxidation state is only accessible for manganese.
