Violation of the Minimum H-H Separation "Rule" for Metal Hydrides

TL;DR

This study uses advanced density-functional calculations to reveal that certain metal hydrides can have H-H separations shorter than the traditionally accepted minimum of 2 Å, challenging established empirical rules.

Contribution

It demonstrates that metal hydrides can violate the minimum H-H separation rule through detailed electronic structure analysis, providing new insights into bonding in these materials.

Findings

H-H separation as short as 1.454 Å observed

Metallic and ionic bonding components identified

Short H-H distances explained by electron polarization and R-R interactions

Abstract

Using gradient-corrected, all-electron, full-potential, density-functional calculations, including structural relaxations, it is found that the metal hydrides $RT$InH$_{1.333}$ (R = La, Ce, Pr, or Nd; T = Ni, Pd, or Pt) possess unusually short H-H separations. The most extreme value (1.454 {\AA}) ever obtained for metal hydrides occurs for LaPtInH$_{1.333}$. This finding violates the empirical rule for metal hydrides, which states that the minimum H-H separation is 2 {\AA}. Electronic structure, charge density, charge transfer, and electron localization function analyses on $RT$InH$_{1.333}$ show dominant metallic bonding with a non-negligible ionic component between T and H, the H-H interaction beingweakly metallic. The paired, localized, and bosonic nature of the electron distribution at the H siteare polarized towards La and In which reduces the repulsive interaction between negatively charged H atoms. This could explain the unusually short H-H separation in these materials. Also, R-R interactions contribute to shielding of the repulsive interactions between the H atoms.