Three new gadolinium(III) complexes with carboxylate ligands of formula [Gd₂(ac)₆(H₂O)₄]·2H₂O (1), [Gd₂(ac)₂(fum)₂(H₂O)₄]_n (2) and [Gd₂(ox)(fum)₂(H₂O)₄]_n·4nH₂O (3) (ac = acetate, fum = fumarate and ox = oxalate) have been prepared and their structures determined by X-ray diffraction on single crystals. The structure of 1 is made up of discrete centrosymmetric di-µ-oxo(carboxylate acetate)digadolinium(III) units with an intramolecular Gd⋯Gd separation of 4.1589(3) Å. Each gadolinium atom in 1 is nine-coordinated with two water molecules and seven carboxylate-oxygen atoms from four acetate ligands building a monocapped square antiprism environment. Compound 2 exhibits a sheet-like structure, the repeating intralayer motif being a rectangle of di-µ-oxo(carboxylate acetate)digadolinium(III) units [Gd⋯Gd separation of 3.8659(10) Å] where the edges are defined by bridging bis-bidentate and tetrakis-monodentate fumarate groups. Each gadolinium atom in 2 is nine-coordinated with two water molecules and seven carboxylate oxygens from two acetate and three fumarate ligands building a distorted monocapped square antiprism. Compound 3 has a three-dimensional structure where chains of double oxo(carboxylate fumarate)-bridged gadolinium(III) ions are interlinked by bis-bidentate fumarate to afford layers which are further interconnected through bis-bidentate oxalate ligands. The corresponding values of the Gd⋯Gd separation are 4.5816(1), 8.2292(2) and 6.2989(1) Å. Each gadolinium atom in 3 is ten-coordinated with two water molecules and eight oxygen atoms from four fumarate ligands and one oxalate group building a distorted bicapped square antiprism. Variable-temperature magnetic susceptibility measurements show the occurrence of weak but significant ferro- (1 and 3) and antiferromagnetic (2) interactions through the double oxo(carboxylate acetate) (1 and 2), double syn-syncarboxylate fumarate (2) and double oxo(carboxylate fumarate) and bis-bidentate oxalate (3) bridges. Monte Carlo calculations were performed in the case of 3 to simulate its magnetic data and to substantiate the best-fit values of the magnetic couplings through the different exchange pathways involved.

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