Source of materialCs 2CO3 (652 mg, 2mmol) was carefully added to an aqueous solution (15 ml) containing maleic acid (464 mg, 4mmol), until no further bubbles formed. The reaction mixture produced acolourless and clear solution which was stirred at 323 Kfor 2huntil it solidified. The solid product was re-dissolved in water (5 ml) and allowed to stand for three days at ambient temperature, after which transparent fine crystals were harvested. Experimental detailsThe H1 atom was positioned geometrically and refined using riding model, with d(C-H) =0.93 Å and U iso (H) =1.2 U eq (C). The H2 atom was found in difference Fourier map and refined. The asymmetric unit of the title compound contains one caesium cation, one hydrogen maleate anion. The Cs atom is tencoordinated by ten Oa toms from seven different hydrogen maleateions. The anion is linked to two caesium cations, while the Cs + cation is surrounded by seven organic ligands, three of which are coordinated by employing both Oatoms and the other four are coordinated solely by Oatoms. The Cs-Odistances are in the range from 3.196 (2) to 3.466 (2) Å,which are well within the range reported in the literature for other caesium complexes [10][11][12]. In the crystal structure, intramolecular hydrogen bonds occur, linking carboxylate Oatoms. The H2 atom is involved in this bond and maintains the charge balance by sharing between two carboxylate groups within the structure.