"Inverted" sodium-lithium electronegativity: polarity and metalation energies of organic and inorganic alkali-metal compounds

Abstract

The polarity and the metalation energies of selected organic and inorganic alkali-metal compounds MR (R = H, CH3, NH2, OH, F, a5-C5H5), studied by ab initio pseudopotential calculations, show two types of behavior, both monotonous and nonmonotonous, along the Li-Cs series. The MX bonds in the NH2, OH, F, and q5-C5H5 derivatives are almost completely ionic, and the Li to Cs trends are monotonous. In contrast, alkali-metal hydrides and methides show some covalent character and a nonmonotonous behavior of the metal charges p along the seriespLi > p~~ < p~ < PRb < pes. This trend, which parallels the Allred-Rochow electronegativities of the alkali metals, is attributed to varying covalent u contributions to the MX bonding. The covalent bonding contributions in MH and MCH3 influence the energies of metalation of the NH3, H20, HF, and C5He set and results in nonmonotonous behavior down group I. In contrast, the metalation energies within one of these sets, e.g. the reaction energies of HP with MCH3 or the reaction energies involving only NH2, OH, F, or C5H5 derivatives, are almost independent of the metal.