Chemical bonding in metal sandwich molecules M_nR₂ with R = pyrene C₁₆H₁₀ and tetracene C₁₈H₁₂

The structures and properties of neutral molecular complexes of palladium transition metal atoms sandwiched between pyrene C₁₆H₁₀ and tetracene C₁₈H₁₂ molecules have been studied using plane wave based density functional theory. Changes in symmetry and variations in the number of metal atom (n = 4, 8 for pyrene and n = 4-9 for tetracene) were considered. The metal atoms prefer edge sites with low coordination to ring carbon atoms over "interior" sites with the high coordination as found in bis(benzene) palladium. This appears as a general motif for small polyacene molecules. All compounds examined had stable singlet (S = 0) ground states relative to separate hydrocarbon and metal cluster, and a lowest triplet state (S = 1) with spin on the metal atoms. When restricted to D_2h symmetry the sandwich Pd₄(C₁₆H₁₀)₂ with pyrene had palladium atoms that were isolated from each other in η²- and η³-coordinated sites. The valence electron density was provided mainly by overlap of metal d_xy- and d_yz-functions with carbon p_y (y-axis ⊥ sandwich plane). The total charge density isometric surfaces showed that η³-coordination involved higher charge density than η²-coordination, and that both were weaker than C-C bond charge by an order of magnitude. In the pyrene n = 8 complex, weak metal-metal bonding was apparent around the molecular perimeter together with some metal bonding to the lone interior Pd atom. In sandwiches with tetracene the metal atoms had η²- and η³-coordination with ring edge carbon atoms. There were no metal atoms on interior sites. The n = 5 compound with a lopsided Pd distribution had a bonding pattern consistent with experimental work of Murahashi et al. [T. Murahashi, M. Fujimoto, M. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki, H. Kurosawa, Science 313 (2006) 1104]. Based on geometry (separation ≤275 pm), total charge density and partial charge density components, we found evidence for metal-metal bonds. In the complex with highest loading (n = 9) the metal-metal bonds encompassed the molecule. The Kohn-Sham partial charge densities of these bonds were identified.