Inverse bilayer structure of mononuclear Co(II) and Ni(II) complexes of the type M(H2O)3(SO4)(4-CNpy)2.

Two new metal compounds of the formula [M(H(2)O)(3)(SO(4))(4-CNpy)(2)] x H(2)O [M = Ni (1) and Co (2), 4-CNpy = 4-cyanopyridine] have been prepared and studied by X-ray diffraction. In both of these compounds the 4-CNpy ligands are coordinated via pyridyl-N atoms to the metal ions in a cis fashion. The neutral complexes along with the uncoordinated H(2)O molecules are glued together preferentially into inverse bilayers by non-covalent interactions, including unique interlayer pi-pi interactions between antiparallel nitrile groups. Hartree-Fock and density-functional theory (DFT) calculations indicate that the pi-pi interactions are energetically significant. The unit-cell similarity index (Pi) of 0.0046 for the compounds suggests their isostructurality, which is also supported by their X-ray powder diffraction patterns that can be almost superimposed.