Synthesis and Chemical Bonding of Lithium and Alkaline-Earth Metal-based Carbometallates



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Organometallic molecules of nickel and palladium are typically employed as catalysts. Carbometallates are promising precursors for the synthesis of such organometallic compounds. Transition metal-to-carbon bonds are generally difficult to form in a typical organometallic synthesis. However in the solid-state compounds, these bonds are already pre-formed. Unfortunately, the vast majority of reported carbometallates are based on rare-earth elements and actinoids which generally have high lattice energies and thus are usually insoluble in most solvents. This solvation problem could hinder the application of carbometallates as precursors for organometallics. To address this issue, new carbometallates were synthesized using only highly polarizing lithium and alkaline-earth elements as counter-cations.
Three new nickel-based quaternary carbometallates (Li2Sr3[Ni(C2)3], Li2Ba3[Ni(C2)3], and Li2Ca2[Ni(C2)2]) are reported in this research. The isotypic compounds, Li2Sr3[Ni(C2)3] and Li2Ba3[Ni(C2)3], crystallize in a new structure-type (hP12) in space group P"6" ̅2m. These are the first representatives in the Li-Sr-Ni-C and Li-Ba-Ni-C quaternary systems. The new carbonickelates feature a discrete trigonal planar Ni(C2)3 unit. Another new carbometallate, Li2Ca2[Ni(C2)2], crystallizes in a new structure-type (tI36) in space group I4/mmm. It is the first representative in the Li-Ca-Ni-C quaternary system. It highlights a unique linear Ni(C2)2 molecular unit. These three carbonickelates are the first examples of molecular carbometallates based on alkali and alkaline-earth elements.
Two new palladium-based compounds, Ca2Pd3C and LiCa2Pd2, were also synthesized albeit these are not carbometallates. The metal carbide Ca2Pd3C crystallizes in space group P63/mmc and is just the second representative in the Ca-Pd-C ternary system. It features a Pd Kagomé net. The palladide, LiCa2Pd2, is a new structure-type (oC40) in space group Cmca. It is the first representative in the Li-Ca-Pd ternary system, and features novel linear Pd chains.