Key indicators: single-crystal X-ray study; T = 200 K; mean (C-C) = 0.004 Å; R factor = 0.044; wR factor = 0.130; data-to-parameter ratio = 13.2.The asymmetric unit of the title compound, C 10 H 24 N 3 + Á-0.5C 4 H 2 O 4 2À Á0.5C 4 H 4 O 4 , comprises a triisopropylguanidinium cation, half of a fumarate dianion and half of a fumaric acid molecule; both the fumarate dianion and the fumaric acid molecule are located on inversion centres. In the crystal, intermolecular O-HÁ Á ÁO hydrogen bonds between the carboxyl groups of the fumaric acid molecules and the carboxylate groups of the fumarate anions lead to the formation of a hydrogen-bonded supramolecular twisted chain along the b axis. The triisopropylguanidinium cations interact with the fumarate-fumaric acid chains via extensive N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, leading to a ladder arrangement, with the cation being the rungs that bridge three curled chains of fumarate-fumaric acid. The crystal packing is stabilized by N-HÁ Á ÁO and C-HÁ Á ÁO (cationÁ Á Áfumarate/ fumaric) and O-HÁ Á ÁO (fumarateÁ Á Áfumaric) hydrogen bonds, consolidating a three-dimensional network.
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