Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refined as a two-component inversion twinThe position of the hydromaleate acidic hydrogen atom was freely refined. The structure was solved from its 0.5 hydrate analogue by isomorphous replacement. Two THF molecules were refined as disordered. One in a 1:1 ratio around a two-fold axis, the other in a general position. The three disordered moieties were restrained to have similar geometries. Uij components of ADPs for disordered atoms closer to each other than 2.0 Angstrom were restrained to be similar. Subject to these conditions the occupancy ratio for the molecule in the general position refined to 0.587 (16) to 0.413 (16).