Some new hydrazine complexes of trivalent lanthanide 3,3’-thiodipropanoates of empirical formulae [M(N₂H₅)(tdp)₂] and [Pr(N₂H₅)(tdp)₂·H₂O], where M = La, Ce, Sm and Nd, H₂tdp = 3,3’-thiodipropanoic acid, have been prepared and characterized by elemental analysis, mass, FTIR, Raman and electronic spectral, powder X-ray diffraction, scanning electron microscopy (SEM) and simultaneous TG–DTA techniques. The presence of bidentate carboxylate anion and coordinated N2H5 in these complexes are confirmed by IR spectra. TGA/DTA results confirmed that all the rare earth metal complexes give the corresponding metal carbonate (heating up to 1000 °C) through a metal oxalate intermediate. SEM images of Pr2O3 residue obtained from its complex show nano-sized clusters, thus the complex may be used as a precursor for nano-Pr₂O₃ preparation. The synthesized complexes show good antimicrobial activity against six bacteria (Bacillus cereus, Staphylococcus aureus, Proteus vulgaris, Pseudomonas aeruginosa, Escherichia coli, and Serratia species) and four fungi (Candida albicans, Aspergillus niger, Aspergillus fumigatus, and Penicillium variance) as assessed by in vitro biological activity.