Bonding and magnetism in transition metal sandwich structures with the aromatic hydrocarbon coronene C₂₄H₁₂ outer layers

Plane wave based ab initio density functional theory has been used to study the chemical bonding and magnetism of transition metal atoms between coronene molecules in sandwich structures M_n(C₂₄H₁₂)₂ where n = 7 and M = Cr, Fe, Pd. Symmetry conditions permit the metal atoms to occupy either a central site with η⁶-coordination as in the metal bis-benzene molecules M₁(C₆H₆)₂ or a lower symmetry edge site with η²- or η³-coordination. In some cases at the edge sites the metal atoms crimped the edges of the sandwich towards the smaller separations of the bis-benzene molecules. However, since this effect also depended on local metal atom spin, the sandwich cross-sections could be concave in one symmetry plane and convex in an orthogonal plane. The lowest energy states of the sandwiches had spin S = 5 (Cr), 6 (Fe) and 0 (Pd). The high spin systems occurred with a larger metal-ring spacing so that a concomitantly weaker electric field crossed the metal atom compared to the bis-benzene molecules which have S = 0 (Cr), 2 (Fe) and 0 (Pd) in the ground electronic state. In the spin polarized ground states the unpaired electrons resided exclusively on the metal atoms. In the Pd sandwich the excited states with spin S = 1 and S = 2 had qualitatively similar spin distributions to the Fe and Cr structures, with the difference that there was a much greater leakage of spin density onto the nearby carbon atoms. In all the sandwiches the spin density was not evenly distributed amongst metal atoms, and depended on the environment of the metal as gauged for example by the metal-nearest ring separation.