Spectroscopic and Ab initio characterization of the [ReH₉] ^2- ion

The dynamics and bonding of the hydrido complex Ba[ReH₉], containing the D_3h face-capped trigonal prismatic [ReH ₉]^2- ion, have been investigated by vibrational spectroscopy and density functional theory (DFT). The combination of infrared, Raman, and inelastic neutron-scattering (INS) spectroscopies has enabled observation of all the modes of the [ReH₉]^2- ion for the first time. We demonstrate that calculations of the isolated [ReH ₉]^2- ion are unable to reproduce the INS spectrum and that the complete unit cell must be considered with periodic DFT to have reliable results. This is shown to be a consequence of the long-range Coulomb potential present. Analysis of the electronic structure shows that the bonding between the rhenium and the hydrogen is largely covalent. There is a small degree of covalency between the prism hydrides and the barium. The counterion is crucial to the stability of the materials; hence, variation of it potentially offers a method to fine-tune the properties of the material.