Synthesis, characterization and thermal investigation of M[M(C2O4)3]•xH2O -: (x = 4 for M = Cr(III); x = 2 for M = Sb(III) and x = 9 for M = La(III))

The complexes, M[M(C2O4)(3)]. xH(2) O, where x=4 for M=Cr(III), x=2 for M=Sb(III) and x=9 for M=La(III) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR and electronic spectral data, conductivity measurement and powder X-ray diffraction (XRD) studies. The chromium(III)tris(oxalato)chromate(III)tetrahydrate (COT), Cr[Cr(C-2 O-4)(3)]. 4H(2)O, released water in a stepwise fashion. Removal of the last trace of water was accompanied by a partial decomposition of the oxalate group. Thermal investigation using TG, DTG and DTA techniques in air produced Cr2O3 at similar to 858 degrees C through the intermediate formation of Cr2O3 and CrC2O4 at around 460 degrees C. While DSC study in nitrogen up to 670 degrees C produced a mixture of Cr2O3 and CrC2O4. In antimony(III)tris(oxalato)antimonate(III)dihydrate (AOD), Sb[Sb(C2O4)(3)]. 3H(2)O the dehydration took place during the decomposition of precursor at 170-290 degrees C and finally at ca. 610 degrees C Sb-2 O-5 along with trace amounts of Sb2O4 were produced. Trace amount of Sb2O3 and Sb along with Sb2O is proposed as the end product at 670 degrees C of AOD in nitrogen. The oxide La2O3 is formed at similar to 838 degrees C from the study with TG, DTG and DTA in air of lanthanum(III)tris(oxalato)lanthanum(III)nonahydrate (LON), La[La(C2O4)(3)]. 9H(2)O. Intermediate dioxycarbonate, La2O2CO3 was generated at similar to 526 degrees C prior to its decomposition to lanthanum oxide in air; whereas in N-2 the formation of La-2(CO3)(3) at similar to 651 degrees C was proposed. The thermal parameters have been evaluated for each step of the dehydration and decomposition of GOT, AOD and LON using five non-mechanistic equations i.e. Flynn and Wall, Freeman and Carroll, Modified Freeman and Carroll, Coats-Redfern and MacCallum-Tanner equations. Kinetic parameters, such as, E*, k(o), Delta H*, Delta S* etc. were also supplemented by DSC studies in nitrogen for all the three complexes. Some of the intermediate species have been identified by analytical and powder XRD studies. Tentative schemes has been proposed for the decomposition of all three compounds in air and nitrogen. (C) 1998 Elsevier Science B.V.