The six-coordinate mononuclear iron(III) complexes [Fe(salpm)(2)]ClO4-0.5EtOH, [Fe(salpm)(2)]Cl, [Fe{(3,5-Bu-t(2))salpm}(2)]X (X = ClO4- or Cl-), and [Fe{(3,5-Bu-t(2))-salpm}(2)]NO3 center dot 2H(2)O [Hsalpm = N-(pyridin-2-ylmethyl)salicylideneamine; H(3,5-Bu-t(2))-salpm = N-(pyridin-2-ylmethyl)-3,5-di-tert-butylsalicylideneamine] have been synthesized and isolated in crystalline form; their chemical identities have been ascertained by elemental analyses, FAB mass spectrometry, and infrared spectroscopy. The room-temperature effective magnetic moments [(8 chi T-M)(1/2) similar to 5.85-5.90 mu(B)] of these complexes are consistent with the high-spin (S = 5/2) ground state. These complexes are intensely colored on account of the strong p(pi)-> d(pi)* LMCT visible absorptions. Definitive evidence for the structures of [Fe(salpm)(2)]ClO4 center dot 0.5EtOH and [Fe{(3,5-Bu-t(2))-salpm}(2)]NO3 center dot 2H(2)O has been provided by single-crystal X-ray crystallography. The monomeric complex cations in both compounds comprise two uninegative phenolate-pyridyl tridentate Schiff-base ligands coordinated meridionally to the iron(Ill) to afford a distorted octahedral geometry with a trans, cis, cis-[FeO2N4] core. Whereas [Fe(salpm)(2)]ClO4 center dot 0.5EtOH undergoes a thermally induced (6)A(1) <-> T-2(2) crossover, [Fe{(3,5-Bu-t(2))-salpm}(2)]NO3-2H(2)O retains its spin state in the solid state down to 5 K. However, EPR spectroscopy reveals that the latter complex does exhibit a spin transformation in solution, albeit to a much lesser extent than does the former. The spin crossover in [Fe(salpm)(2)]ClO4 center dot 0.5EtOH has resulted in an unprecedented crystallographic observation of the coexistence of high-spin and low-spin iron(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct ferric centers are both high spin; however, one [Fe(salpm)(2)](+) complex cation undergoes a complete spin transition over the temperature range similar to 200-100 K, whereas the other converts very nearly completely between 100 and 65 K; similar to 10% of the complex cations in [Fe(salpm)(2)]-ClO4 center dot 0.5EtOH remain in the high-spin state down to 5 K.