All the materials were reagent grade, obtained from commercial sources and used without purification. Elemental analyses were performed on a Vario-EC III analytical instrument. The melting points were obtained with LABINDIA-Visual melting range apparatus and were uncorrected. IR spectra in the 4000 - 400 cm⁻¹ range were measured with a JASCO-320 FT-IR spectrometer using KBr pellet technique. Thermal analyses (under Nitrogenated atmosphere, heating rate of 10˚C/min) were carried out in a Perkin-Elmer SII apparatus. The absorption spectra were recorded on a Perkin-Elmer UV-Vis spectrophotometer, JASCO-FP 630. The XRD spectra were recorded on a Bruker AXS D8 advanced X-ray diffractometer using Cu-kα radiation with nickel filter. Photoluminescence (PL) spectra were obtained on a JASCO-FP 6600 spectroflurometer. The excitation and emission spectra were recorded at ambient temperature.

The respective metal oxides (e.g., 0.163 g of La₂O₃ 0.001 mol) were dissolved in a 2 N∙HNO₃ and evaporated to dryness. To this residue, 30 mL of distilled water was added. An aqueous solution (25 mL) of aminoguanidine bicarbonate (0.004 mol, 0.544 g) was neutralized with thiodipropinic acid (0.004 mol, 0.712 g). To this ligand solution was added slowly to metal nitrate solution with constant stirring. To dissolve the precipitate and to monitor the pH, 2 or 3 drops of 2 N∙HNO₃ was added. The resulting solution of pH 4 - 6 was concentrated on a water bath to one fourth of its volume. During evaporation, the polycrystalline solids were obtained and which was kept aside at room temperature for a day. The next day, the product was separated out and washed with cold water or ether and air dried. Physico-chemical techniques are the same as described [8] .

All the compounds are obtained poly crystalline solids, which are stable in air and insensitive to light. They are soluble in water and insoluble in common organic solvents like ethanol, acetone and chloroform.

The analytical data of the complexes are compatible with the proposed composition for the complexes (Table 1). The general reaction for the formation of the aminoguanidine lanthanide thiodipropionate hydrates may be written as follows.

where, Ln = La, Pr, Nd and Sm; if n = 2.

Further, it is observed that the 1:4:4 ratio of lanthanide nitrate, thidipropionic acid and aminoguanidine bicarbonate, respectively, did not yield any desired product, instead only lanthanide thiodipropionate were obtained.

^*Reported value; d: decomposition temperature.

The N-N stretching frequency of the free aminoguanidine is known to occur in the region 1113 cm⁻¹ [9] . The N-N stretching frequencies (Table 2) observed in the range 1110 - 1139 cm⁻¹ is an ample evidence for the presence of coordinated aminoguanidine as a neutral in the complexes. The carbonyl stretching of free acid is observed at 1696 cm⁻¹. In all the complexes the asymmetric and symmetric stretching frequencies of the carboxylate ions are seen in the range 1560 - 1540 cm⁻¹ and 1460 - 1440 cm⁻¹, respectively, with an average separation of (∆v = v_asym − v_sym) 100 cm⁻¹ indicating the bidentate coordination of both carboxylate groups [10] in the dianion. All the complexes exhibit a strong band in the region of 650 - 760 cm⁻¹ due to C-S stretching of the thio- ether linkage. All the compounds exhibit strong bands in the range of 3460 - 3430 cm⁻¹ due to O-H stretching, confirming the presence of water molecules [10] .

The absorption spectra of the Pr, Nd and Sm were recorded and compared with the data for the corresponding aqua-ions. The spectral profiles of the complexes show not only shifts in positions, but also changes in intensity compared to those of the aqua-ions (Figure 1), i.e., the energies at which the various bands appear are lower. This red shift, which is measure of metal-ligand covalent binding, has been ascribed to the nephelsauxetic (cloud expanding) effect [11] . The absorption band associated with nearly degenerate ⁴I_9/2∙²G_7/2, ⁴G_5/2 transition of Nd³⁺ is known to exhibit strong hypersensitivity behavior [12] , making it especially suitable for probing the coordination environment around the Nd³⁺ ion.

All the parameters β, b^1/2, % δ and η values (Table 2) are comparable with those of oxygen-donor ligands such as oxalate and phthalate [12] -[14] . The β values obtained in the in series are around unity showing an almost completely ionic character for the Ln³⁺ ligand interaction. The spectral profiles present Nd³⁺ complex resembles those of the nine-coordinated complexes [15] [16] .

The simultaneous TG-DTA results of the La, Pr, Nd and Sm complexes are similar and all of them under go three stages of weight loss (Table 3) upon heating. The first stage, which occurs in the range 80˚C - 200˚C is attributed to the loss of two water molecules. In DTA this loss of water is observed as an endotherm around 200˚C and such slightly high temperature of dehydration confirming the presence of two coordinated water molecules in the complexes. This loss of water is supported by the weight loss in TG. The presence of lattice and coordinated water molecules has been reported [16] . After this the anhydrous compounds loss aminoguanidine exothermically around 200˚C - 380˚C to form Ln₂(tdp)₃ as an intermediate. In DTA, this loss of two molecules each of aminoguanidine seen as a broad exotherm around 380˚C. In the final step, the isomeric thiodipropionates of metal decompose exothermically in the range 380˚C - 800˚C to give the respective Ln₂S₃ as the final residue.

As a representative example, the decomposition reaction of the lanthanum compound, in air are given below on the basis of the mass losses in the TG curve.

The simultaneous TG-DTA of the La, Pr, Nd, and Sm compounds are shown in Figure 2. Our effort to isolate the intermediates was unsuccessful due to their continuous decomposition as evident from the TG. Hence, the probable intermediates have been assigned based on the calculated mass losses.

where Ln = La, Pr, Nd and Sm.

The X-ray powder diffraction patterns for the complexes are shown in Figure 3. From the patterns, it is evident that the La, Pr, Nd and Sm complexes are isostructural. The results IR and thermo analytical studies are in accordance with these isomorphism structural groups.

Lanthanide luminescence is demonstrated to be very sensitive to local environments around the lanthanide center [17] . Figure 4 shows the absorption spectra cover the wavelength range in the visible area. The luminescent properties of the three compounds were measured at room temperature. The Ex. Slit and Em. Slit are 2.5 and 5 nm respectively. The shape of UV absorption spectrum and excitation spectrum is basically similar, in addition to the different spectral intensity. The optimum excitation wavelength is around 400 to 590 nm for complexes (a) and (b), respectively, which is mainly derived from the π-π transition absorption of ligand [18] .

The praseodymium complex shows the light emission upon excitation at 589 nm. The characteristic luminescent band of the PrIII complex has been recorded at 652 nm, which can be attributed ³H₄∙¹D_J, ³P_J (J = 0, 1, 2) shown in Table 2.

The neodymium complex shows the light emission upon excitation at 427 nm and 577 nm at room temperature. This displays photoluminescence at 499 nm, 638 nm respectively, which can be attributed to ⁴I_9/2∙⁴F_J, ²H_J, ⁴G_J shown in Table 2, transitions takes place.

The samarium complex shows the light emission upon excitation at 402 nm at room temperature. The luminescent band of the SmIII complex is observed at 466 nm, which can be attributed to ⁴G_5/2∙⁶H_J transition.

These characteristic emission bands indicate that the ligand to metal transfer is moderately efficient under the experimental conditions used. These characteristic emission bands indicate that the ligand-to-metal energy transfer is moderately efficient under the experimental conditions used [19] .