Hydrogen bonding and π-π interactions in the cocrystal salt [Fe(bpe)2(H2O)4](TCEP)2·2(bpe) [bpe is trans-1,2-bis(pyridin-4-yl)ethene and TCEP is 1,1,3,3-tetracyano-2-ethoxypropenide].

The cocrystal salt tetraaquabis[trans-1,2-bis(pyridin-4-yl)ethene-κN]iron(II) bis(1,1,3,3-tetracyano-2-ethoxypropenide)-trans-1,2-bis(pyridin-4-yl)ethene (1/2), [Fe(C12H10N2)2(H2O)4](C9H5N4O)2·2C12H10N2, is a rare example of a mononuclear FeII compound with trans-1,2-bis(pyridin-4-yl)ethane (bpe) ligands. The complex cation resides on a crystallographically imposed inversion center and exhibits a tetragonally distorted octahedral coordination geometry. Both the symmetry-independent bpe ligand and the cocrystallized bpe molecule are essentially planar. The 1,1,3,3-tetracyano-2-ethoxypropenide counter-ion is nonplanar and the bond lengths are consistant with significant electron delocalization. The extended structure exhibits an extensive O-H...N hydrogen-bonding network with layers of complex cations joined by the cocrystallized bpe. Both the coordinated and the cocrystallized bpe are involved in π-π interactions. Hirshfeld and fingerprint plots reveal the important intermolecular interactions. Density functional theory was used to estimate the strengths of the hydrogen-bonding and π-π interactions, and suggest that the O-H...N hydrogen bonds enhance the strength of the π-interactions by increasing the polarization of the pyridine rings.