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The enthalpies of solution of neodymium(III) and erbium(III) succinates and malates in the system HCl . 100H 2 O were determined and used to calculate the standard enthalpies of formation of these salts.The available information on the enthalpies of formation of complexes of rare-earth metals with dicarboxylic acids is contradictory and insufficient [1,2]. Previously we determined [2] standard enthalpies of formation for lanthanum(III), samarium(III), and holmium(III) succinates. In the present work we have studied the thermodynamic properties of neodymium(III) and erbium(III) succinates and malates.We prepared dehydrated complexes of rare-earth metals with succinic and malic acids, such as ) 3 . The IR spectra of carboxylic acids contain absorption bands at 175031680 cm !1 , characteristic of protonated carboxy groups, while the spectra of neodymium(III) and erbium(III) succinates and malates lack these bands and show bands at 150031590 cm !1 characteristic of asymmetric vibrations of COO ! groups, which suggests formation of neutral salts [3]. The complexes of neodymium(III) and erbium(III) malates contain two bands at 10803 1010 cm !1 , that are lacking in the spectra of malic acid and can be assigned, according to [4], to stretching vibrations of the OH ! group coordinated with metals.The enthalpies of solution of solid rare-earth complexes in the HCl . 100H 2 O (DH 1 ) solution were determined by calorimetry. From these data, using the thermochemical cycle described in [2], we calculated the standard enthalpies of formation of dehydrated neodymium(III) and erbium(III) succinates and malates (see table). The resulting DH 5 values for neodymium(III) and erbium(III) succinates agree with the standard enthalpies of formation of other rare-earth metal succinates, reported in [2]. The standard enthalpies of formation of neodymium(III) and erbium(III) malates are more exothermic, which is probably connected with the nature of the ligand [3, 5]. EXPERIMENTAL The IR spectra were registered in an Infralyum FT-02 instrument at 40003350 cm !1 for KBr pellets. Neodymium(III) and erbium(III) succinates and malates were prepared by pouring together solutions of rare-earth metal chlorides (c 0.25 M) and alcoholic Standard enthalpies of formation of neodymium(III) and erbium(III) succinates and malates, and values used for their calculation (DH, kJ mol !1 ) a ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄ a (DH 1 ) Enthalpies of solution of succinates and malates; (DH 2 ) enthalpies of solution of rare-earth metals in HCl .100H 2 O; (DH 3 ), enthalpies of solution of acids in HCl .100H 2 O; (DH 4 ) standard enthalpies of formation of acids; and (DH 5 ) standard enthalpies of formation of complexes.

Heterometallic Nd(III) and Fe(III) complexes with succinic, malic, and tartaric acids were synthesized and studied by IR spectroscopy, thermography, and dissolution calorimetry. The formation of heterometallic compounds with specific properties was confirmed by comparison of physicochemical characteristics of heteronuclear and the corresponding mononuclear carboxylates.

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