Hydrides: Solid State Transition Metal Complexes

AbstractAt present, some 50 different homoleptic transition (T) metal hydride complexes are known to occur in over 127 compounds that cover 47 structure types. The complexes form with T elements from group seven (Mn) to closed d‐shell elements of group twelve (Zn). While the great majority are centred by one T‐metal atom and display up to nine terminal hydrogen ligands, some are centred by two (Ru, Pt), four (Rh, Ni), or more (Ru, Rh) and display both terminal and bridging hydrogen ligands, and/or TT metal bonds. The structural and electronic configurations are consistent with those usually found in coordination compounds. The complexes are stabilized by charge transfer from the surrounding cation matrix, are often 18‐electron, and less often 16‐ or 14‐electron. Some hydride structures show evidence for metal–metal interactions as in typically interstitial metal hydrides while others contain additional hydride anions (H−) bonded to electropositive metals only, such as in saline hydrides. No HH bond formation is observed. Most hydrides are nonmetallic and many are colored. Some, however, show hydrogenation induced complex formation and metal‐to‐insulator transitions. The systems generally display very high volume efficiencies for hydrogen storage but are thermally too stable and/or too expensive for practical applications. Thermodynamic data suggest that their enthalpy of formation scales with the thermal stability of the binary hydrides of the electropositive metal constituents.