Introduction Because of their remarkable ligation properties and sim- ple preparation, semicarbazones represent a very inter- esting group of ligands to coordination chemists [1, 2]. Besides, some of these compounds possess biological activity, although generally lower in comparison with thiosemicarbazones [3]. From the point of view of the biological activity, the most known semicarbazone seems to be 5-nitrofurfural semicarbazone, which under the commercial name Furacillin ® is used as a bactericide [4]. One of the new semicarbazone ligands that has re- cently become the subject of our interest is a tridentate ONO pyridoxal semicarbazone, PLSC. We reported the synthesis and structure of the square-pyramidal [Cu(PLSC)Br 2 ] complex [5] and physico-chemical (magnetic, spectral and voltammetric) characteristics of some Fe(III) complexes [6, 7], among them being one of the title complexes, viz. [Fe III (PLSC)(H 2 O)Cl 2 ]Cl. In the present paper we describe the crystal and molecular structure and thermal decomposition of this compound together with the structural and thermal characteristics of the newly-prepared [Fe II (PLSC)(H 2 O) 2 SO 4 ] com- plex. The discussion of the thermal decomposition is mainly focused towards the deaquation process in view of the crystal structure, emphasizing the complexity of the related parameters. The decomposition processes above 600 K, especially in lack of coupled measure- ments, are too uncertain to be discussed. Experimental Preparation of the compounds Green single crystals of [Fe II (PLSC)(H 2 O) 2 SO 4 ], 1, were obtained by mixing of solutions of FeSO 4 with aqueous suspension of PLSC×2H 2 O in a molar ratio of 1:1. The mixture was heated until complete dissolu- tion of the ligand. Yield: 58%. Elemental analysis data, %, calcd. (found): C, 26.22 (26.11); H, 3.11 (2.97); N, 13.60 (13.54). The ligand, PLSC×2H 2 O (Scheme 1) and [Fe III (PLSC)(H 2 O)Cl 2 ]Cl, 2, were synthesized ac- cording to the previous procedures [6]. 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 90 (2007) 2, 549–555 STRUCTURAL AND THERMAL CHARACTERIZATION OF Fe(III) AND Fe(II) COMPLEXES WITH TRIDENTATE ONO PYRIDOXAL SEMICARBAZONE ¦. K. Jaºimoviº 1 , V. M. Leovac 2 , G. Giester 3 , Z. D. Tomiº 4 and Katalin Mészáros Szécsényi 2* 1 Faculty of Metallurgy and Technology, University of Montenegro, Podgorica, Montenegro 2 University of Novi Sad, Faculty of Sciences, Department of Chemistry, Novi Sad, Serbia 3 Institut für Mineralogie und Kristallographie, Universität Wien, Wien, Austria 4 ‘Vin¹a’ Institute of Nuclear Sciences, Laboratory of Theoretical Physics and Condensed Matter Physics, P.O. Box 522 11001 Belgrade, Serbia Molecular and crystal structure of diaqua-sulphato(pyridoxal semicarbazone-ONO)iron(II) and dichloroaqua(pyridoxal semicarbazone- ONO)iron(III) complexes with tridentate ONO pyridoxal semicarbazone ligand, PLSC, of formula [Fe II (PLSC)(H 2 O) 2 SO 4 ] and [Fe III (PLSC)(H 2 O)Cl 2 ]Cl is reported. Thermal decomposition of the compounds was followed in argon and air gas carriers. The de- aquation mechanism is discussed in the view of the crystal structure, emphasizing the complexity of the related parameters and processes. Keywords: crystal structure, deaquation mechanism, iron(II) and iron(III) complexes, pyridoxal semicarbazone thermal decomposition Scheme 1 * Author for correspondence: mszk@uns.ns.ac.yu