Highly polar bonds and the meaning of covalency and ionicity—structure and bonding of alkali metal hydride oligomers

Abstract

The hydrogen–alkali metal bond is simple and archetypal, and thus an ideal model for studying the nature of highly polar element–metal bonds. Thus, we have theoretically explored the alkali metal hydride monomers, HM, and (distorted) cubic tetramers, (HM)₄, with M = Li, Na, K, and Rb, using density functional theory (DFT) at the BP86/TZ2P level. Our objective is to determine how the structure and thermochemistry (e.g., H–M bond lengths and strengths, oligomerization energies, etc.) of alkali metal hydrides depend on the metal atom, and to understand the emerging trends in terms of quantitative Kohn–Sham molecular orbital (KS-MO) theory. The H–M bond becomes longer and weaker, both in the monomers and tetramers, if one descends the periodic table from Li to Rb. Quantitative bonding analyses show that this trend is not determined by decreasing electrostatic attraction but, primarily, by the weakening in orbital interactions. The latter become less stabilizing along Li–Rb because the bond overlap between the singly occupied molecular orbitals (SOMOs) of H˙ and M˙ radicals decreases as the metal ns atomic orbital (AO) becomes larger and more diffuse. Thus, the H–M bond behaves as a text-book electron-pair bond and, in that respect, it is covalent, despite a high polarity. For the lithium and sodium hydride tetramers, the H₄ tetrahedron is larger than and surrounds the M₄ cluster (i.e., H–H > M–M). Interestingly, this is no longer the case in the potassium and rubidium hydride tetramers, in which the H₄ tetrahedron is smaller than and inside the M₄ cluster (i.e., H–H < M–M).