Titanium Hydride Complex BaCa₂Ti₂H₁₄ with 9-Fold Coordination

We present the high-pressure synthesis and crystal structure of a novel titanium hydride complex, BaCa₂Ti₂H₁₄, with 9-fold coordination. It comprises a unique dinuclear [Ti₂H₁₄]^6- complex that consists of a pair of Ti⁴⁺ ions each coordinated by nine hydrides in the monocapped square antiprism, distinguished from the known 9-fold coordination in the mononuclear tricapped trigonal prism of [MH₉]. The dinuclear hydride complex is stabilized by three-center two-electron bonding at the four bridging Ti-H-Ti bonds to compensate for the lack of valence electrons in the Ti⁴⁺ ions. Optical measurements show that BaCa₂Ti₂H₁₄ is a band insulator with a wide band gap of 2.25 eV. Density functional theory calculations reveal that the top of the valence band is dominated by H-1s-derived states, as expected from the 9-fold coordination, which would present a playground for electronic properties such as high-T_c superconductivity when doped with hole carriers or under high pressure.