A series of iron(III) complexes based on the tetradentate ligand 44(1-methyl-1H-imidazol-2-yl)methyl)-1-thia-4,7-diazacyclononane (L) has been synthesized, and their solution properties investigated. Addition of FeCl3 to methanol solutions of L yields [LFeCl2]FeCl4 as a dark red solid. X-ray crystallographic analysis reveals a pseudooctahedral environment around iron(III) with the three nitrogen donors of L coordinated facially. Ion exchange reactions with NaPF6 in methanol facilitate chloride exchange resulting in a different diastereomer for the [LFeCl2](+) cation. X-ray analysis of [LFeCl2]PF6 finds meridional coordination of the three nitrogen donors of L. Electrochemical studies of [LFeCl2](+) in acetonitrile display a single Fe(III)/(II) reduction potential at -280 mV versus ferrocenium/ferrocene. In methanol, a broad cathodic wave is observed because of partial exchange of one chloride for methoxide with half-potentials of -170 mV and -440 mV for [LFeCl2](+/0) and [LFeCl(OCH3)](+/0), respectively. The equilibrium constants for chloride exchange are 7 x 10(-4) M-1 for Fe(III) and 2 x 10(-8) M-1 for Fe(II). In aqueous solutions chloride exchange yields three accessible complexes as a function of pH. Strongly acidic conditions yield the aqua complex [LFeCl(OH2)](2+) with a measured pK(a) of 3.8 +/- 0.1. Under mildly acidic conditions, the mu-OH complex [(LFeCl)(2)(OH)](3+) with a pK(a) of 6.1 +/- 0.3 is obtained. The mu-oxo complex [(LFeCl)(2)(O)](2+) is favored under basic conditions. The diiron Fe(III)/Fe(III) complexes [(LFeCl)(2)(OH)](3+) and [(LFeCl)(2)(O)](2+) can be reduced by one electron to the mixed valence Fe(III)/Fe(II) derivatives at -170 mV and -390 mV, respectively. From pH dependent voltammetric studies, the pK(a) of the mixed valent mu-OH complex [(LFeCl)(2)(OH)](2+) is calculated at 10.3.