A second monoclinic polymorph of ferrocenecarboxaldehyde

The title compound, [Fe(C 5 H 5 )(C 6 H 5 O)] ( I ), crystallizes in the space group P 2 1 with two crystallographically independent ferrocenecarboxaldehyde molecules 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 molecules into infinite chains along the b -axis direction. The π–π stacking interactions between the parallel ferrocene rings [centroid-to-centroid distances of 3.305 (4) and 3.293 (4) Å] and the C—H...π(ring) interactions 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 P 2 1 2 1 2 1 with one molecule 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%) interactions. The volume of the crystal voids and the percentage of free space were calculated to be 53.38 Å 3 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 interactions are the dominant interactions in the crystal packing.