NaFeH and NaCoH: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis.

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH-TM-H at 10 GPa. NaFeH and NaCoH feature pentagonal bipyramidal FeH and octahedral CoH 18-electron complexes, respectively. At high pressure, high temperature (300 < ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics. Upon cooling, subtle changes in the diffraction patterns evidence reversible and rapid phase transitions associated with ordering of the complexes. During decompression, NaFeH and NaCoH transform to tetragonal and orthorhombic low pressure forms, respectively, which can be retained at ambient pressure. The discovery of NaFeH and NaCoH establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.