By using a hindered tripodal ligand, hydrotris(3-tert-butyl-5-isopropylpyrazo-1-y)borate HB(3-tBu-5-iPrpz)(3), a series of monomeric ferrous complexes having acetate, hydroxide, and benzoylformate ligands were synthesized. Reaction of KHB(3-tBu-5-iPrpz)(3) with anhydrous Fe(OAc)(2) yielded acetato complexes Fe(OAc)[HB(3-tBu-5-iPrpz)(3)] (1) and Fe(OAc)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBup (2). A hydroxo complex Fe(OH)[HB(3-tBu-5-iPrpz)(3)] (3) was prepared by the treatment of 1 or 2 with aqueous NaOH. The geometry of Fe(II) in 3 is a slightly distorted tetrahedron as determined by X-ray crystallography. The hydroxo complex 3 reacted with benzoylformic acid to give the benzoylformato complex Fe(O2CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)] (4), which showed thermochromism which depended on the coordination geometry of the benzoylformate ligand. The Fe(LT) ion in the colorless form of 4 isolated at 4 degrees C is coordinated by a tetrahedral N3O1 ligand donor set including the unidentate benzoylformato ligand. On the other hand, the bluish purple form of 4 isolated ar -20 degrees C has a five-coordinate trigonal bipyramidal Fe(IT) center. The benzoylformate ligand in this bluish purple form works as a chelate ligand through coordination of the unidentate carboxylate oxygen atom as well as the ketonic oxygen atom. A benzoylformato complex containing an additional pyrazole, Fe(O2CC(O)Ph)[HB(3-tBu-5-iPrpz)(3)](3-iPr-5-tBupzH) (5), was obtained by the reaction of 3 with benzoylformic acid in the presence of 3-tert-butyl-5-isopropylpyrazole. The iron atom in 5 is coordinated by an N4O1 ligand donor set with trigonal bipyramidal geometry. A hydrogen-bonding interaction between the carboxylate oxygen atom and the additional pyrazole's NH proton in 5 is suggested from the short distance between O-carboxylate and N-pyrazole observed in the X-ray structure and the absence of the nu NH vibration in the LR spectrum.