ABSTRACT Two types of tridentate salicylaldimines viz. N-(pyridin-2-ylmethyl)-3,5-R salicylideneamine [R = H (Hpmsal) or tert-butyl (Hpmbsal)] and N-(pyridin-2 ylamino)-3,5-R-salicylideneamine [R = H (Hpasal), tert-butyl (Hpabsal) or CI (Hpacsal)], were synthesised and characterised. These hetero-donor-atom polydentate ligands were selected carefully in the hope that their complexes would exhibit unique structural, magnetic and spectroscopic features. From these ligands a series of mononuclear six-coordinate iron(III) compounds were synthesised, namely: [Fe(pmsal)2]X, Fe(pmbsal)2]X, Fe(pasal)2]X, Fe(pabsal)2]X and Fe(pacsal)2]X (X = CI, C104 or NO3). The chemical compositions of these compounds were determined by mass spectrometry and elemental analyses. Their physicochemical properties were investigated using infrared spectroscopy, Mössbauer spectroscopy, X-ray crystallography and electronic spectroscopy. The X-ray crystal structure of [Fe(pmsal)2]C104 shows two crystallographically independent complex cations, two perchlorate counter ions and one ethanol solvate to give the empirical formula {[Fe(pmsal)2]C104}2:EtOH. The Fe" coordination sphere in both complex cations is pseudo-octahedral, with the phenolate oxygen atoms in cis positions and the pyridyl nitrogen atoms also in cis positions whereas the imine nitrogens are trans to each other. The donor atoms of each ligand are arranged around the iron centre in a meridional fashion. The complex cations have been designated complex cation 1 and complex cation 2. The two tridentate ligands in the complex are nearly perpendicular to each other. The average bond distances Fe-Ophenolate (1.922 Å), Fe-Npyridine (2.168 Å) and Fe-Nimine (2.098 Å) for complex cation 1 compare favourably with those of related high-spin Fe" octahedral complexes, whereas for complex cation 2, the average bond distances Fe-Ophenolate (1.867 A), Fe-Npyridine (2.034 Å) and Fe-Nimine (1.980 Å) are typical for low-spin Fe" octahedral complexes. The average Fe-Ligand (FeN402) distance in complex cation 1 (1.960 Å) is 0.103 Å shorter than that observed for complex cation 2 (2.063 A), pointing to different spin states for the two complex cations. Complex cation 1 is high spin whereas complex cation 2 is low spin. The spin transition temperature (T.) appears to be - 150 K, at which the two spin states coexist in nearly equal proportions. Variable-temperature Mössbauer spectra for the compound {[Fe(pmsal)21C104XzEtOH were recorded over the temperature range 300–78 K, At room temperature, the complex is high spin. However, on cooling, a gradual spin transition was observed. At 78 K, the complex was predominantly low spin. On the other hand, the Mössbauer spectra for the compound (Fe(pasal)2]CIO4 recorded at 300 and 78 K were independent of temperature, implying that this complex was unable to undergo a spin crossover. Due to time constraints, the other complexes have not yet been characterised by Mössbauer spectroscopy.