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.
Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Naturvetarvägen 14, Box 124, SE-22100 Lund, Sweden. simeon.ponou@chem.lu.se and Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstrasse 30, 48149 Münster, Germany.
Siméon
S
Ponou
Centre for Analysis and Synthesis, Department of Chemistry, Lund University, Naturvetarvägen 14, Box 124, SE-22100 Lund, Sweden. simeon.ponou@chem.lu.se.
Sven
S
Lidin