A second monoclinic polymorph of ferrocenecarboxaldehyde.

The title compound, [Fe(C5H5)(C6H5O)] (I), crystallizes in the space group P21 with two crystallographically independent ferrocenecarboxaldehyde mol-ecules in the asymmetric unit, in which the C-O bond lengths and also the O-C-C bond angles of the carboxaldehyde moieties have significantly different values. In the crystal, C-H⋯O hydrogen bonds link the mol-ecules into infinite chains along the b-axis direction. The π-π stacking inter-actions between the parallel ferrocene rings [centroid-to-centroid distances of 3.305 (4) and 3.293 (4) Å] and the C-H⋯π(ring) inter-actions help to consolidate the packing. Compound I is a polymorph of the previously reported form of ferrocenecarboxaldehyde [Sato et al. (1984 ▸). Bull. Chem. Soc. Jpn 57, 634-638; Lousada et al. (2008 ▸). J. Phys. Chem. A. 112, 2977-2987], which crystallizes in the space group P212121 with one mol-ecule in the asymmetric unit. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H (54.8%), H⋯C/C⋯H (26.5%) and H⋯O/O⋯H (18.4%) inter-actions. The volume of the crystal voids and the percentage of free space were calculated to be 53.38 Å3 and 6.03%, showing that there is no large cavity in the crystal packing. Hydrogen bonding, π-π, C-H⋯π(ring) and van der Waals inter-actions are the dominant inter-actions in the crystal packing.