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A single-crystal neutron-diffraction study of the structure of potassium oxalate monohydrate has been made. The structure consists of oxalate ions hydrogen bonded by the water molecules into infinite linear chains which are held together by electrostatic interactions with the potassium ions. The oxalate ion is planar, and the H–O–H angle of the water molecule is 107.9°±4.3°. The O–H distance is 0.963±0.029 Å in a hydrogen bond of length 2.744±0.017 Å, and the O–H···O angle is 169.1°±2.6°. The coordination of the water molecule is unusual with the two hydrogen atoms and two potassium ions lying almost in a plane around the water oxygen atom. A classification of the hydrogen-bonded water molecules in hydrates with respect to their lone-pair coordination reveals that K2C2O4·H2O may be the only reported structure of the type in which neither of the lone pairs of the hydrogen-bonded water molecule is specifically directed despite the availability of metal ions in the structure.

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