  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.
