The molecular structure of the title compound showing the atomic labeling. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen bonds are shown as dashed lines. The asymmetric unit contains one half of a fumarate dianion, which is disordered over two positions. The other half of the inversion-generated fumarate dianion is shown. Symmetry code: (i) 2 − x, 1 − y, −z.