Hypervalent Bonding in Alkali-Metal Cyanides Detected by High-Temperature Mass Spectrometry.

Abstract

We present a combined experimental and theoretical study on the nature of hypervalent bonding in such molecules as Li2CN, Na2CN, K2CN, and Li2OH. The molecules M2CN (M=Li, Na, K) were observed in the vapor over a mixture of alkali metals and sodium cyanide by high-temperature Knudsen effusion mass spectrometry and Li2OH was detected by time-of-flight mass spectrometry for a molecular beam generated by laser ablation of slightly oxidized lithium metal. Despite their unusual stoichiometries, these molecules were confirmed to be thermodynamically more stable than the corresponding octet molecules (MCN, LiOH). Computational geometry optimization gives four possible structural isomers to each M2CN molecule; i.e., two planar structures with Cs symmetry and two linear structures with C∞v symmetry. The planar M2CN molecules are favored and best described as a complex of the CN- anion with the M2+ cation. The extra valence electron in SOMO contributes to M-M bonding to form the M2+ unit. The linear M2CN molecules are “electronomers” described as M+(CN)-M· and M·(CN)-M+. The Li2OH molecule with C2v symmetry comprises the Li2+ and OH- units, and the electrostatic attraction affords the stable molecule, similar to the planar M2CN.