Titanium Hydride Complex BaCaTiH with 9-Fold Coordination.

We present the high-pressure synthesis and crystal structure of a novel titanium hydride complex, BaCaTiH, with 9-fold coordination. It comprises a unique dinuclear [TiH] complex that consists of a pair of Ti 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 [MH]. 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 Ti ions. Optical measurements show that BaCaTiH 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- superconductivity when doped with hole carriers or under high pressure.