Enhanced Dihydrogen-Metal Interaction in Transition Metal Exposed Paddle-Wheel Frameworks

The experimentally observed enhancement of hydrogen adsorption in Cu$_{2}$-tetracarboxylate paddle-wheel frameworks is investigated by first-principles density-functional theory calculations [1]. We reveal that the puzzling enhancement is due to the effective orbital coupling between the occupied H$_{2}$ \textit{$\sigma $} and the unoccupied Cu 4$s$-derived states. The nontrivial dihydrogen-metal \textit{$\sigma $s} interaction is enabled by a strong localization of the Cu 4$s$ orbital after hybridizing with the neighboring oxygen 2$p$ orbitals. Based on this understanding, we predict that the dihydrogen-metal interaction can be further increased by alloying Cu with $s$-orbital element Zn or Mg. We will also discuss on the enhanced dihydrogen adsorption on other 3$d$-transition-metal paddle wheel frameworks.\\[4pt] [1] Y.-H. Kim, J. Kang, and S.-H. Wei, Phys. Rev. Lett., in press (2010).