Exo-bonded six-membered heterocycle in the crystal structures of RE₇Co₂Ge₄ (RE = La-Nd)

In this article four new ternary phases of RE₇Co₂Ge₄ (RE = La-Nd) are reported. They were synthesized by a solid-state reaction of the elements at high temperature, and their crystal structures were investigated by single crystal X-ray diffraction data. The isostructural phases crystallize in a new monoclinic structure type, space group P2₁/c, Z = 4, Pearson code mP52, and Wyckoff sequence e¹³. They feature planar {Co₄Ge₆} clusters consisting of a benzene-like six-membered (Co₄Ge₂) metal heterocycle and four Co-Ge exo-bonds. Band structure calculations were carried out to investigate the electronic structures of La₇Co₂Ge₄, in order to understand the nature of the chemical bonding, and the structure directing factors that determine the atomic ordering and the structure stability. These calculations indicate that the cobalt mid-transition metals are present as (formally) anionic species. They also reveal significant valence electron back-donation from the anionic {Co₄Ge₆} units to the La-d orbitals through multicenter bonding involving all atoms of the structure, which is key for the structure's stability with respect to incomplete charge transfer. The applicability of the 18 - n rule in rationalizing the Co-Co relative bond strength was also assessed and, the impact of these metal-metal interactions in stabilizing these clusters was described. Similar valence electron back-donation to stabilize further unusual cluster shapes is expected in other rare earth-transition metal-main group phases.