Hydrides: Solid State Transition Metal Complexes

Abstract

At 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.

Ternary Ruthenium Complex Hydrides for Ammonia Synthesis

“…from LiH-promoted Fe metal catalyst can undergo stoichiometric reaction with dinitrogen forming NH2-containing clusters 20 . This finding inspired us to investigate whether the ternary complex hydrides of alkali (or alkaline earth) and transition metals, a series of compounds belonging to solid-state complexes that often studied in hydrogen storage 21 , would have the potential to catalyze ammonia formation from N2 and H2. The synthesis of solid-state Li4FeH6 requires extremely high pressure of 6.1 GPa 22 , but its Ru analogues are stable entities and can be facilely prepared under normal condition 23 .…”

An Unprecedented Fully H–‐Substituted Phosphate Hydride Sr5(PO4)3H Expanding the Apatite Family

“…Comparison of the residual values for Sr 5 (PO 4 ) 3 D and a possible hydroxide Sr 5 (PO 4 ) 3 OD favor the model containing exclusively deuteride on the Wyckoff site 2 e . As a consequence of the similar ionic radii of hydride and fluoride, the cell parameters derived from X‐ray analysis are very close for X = H, D and F. The strontium to deuterium distance is 2.40 Å, which is in the range of known metal hydrides , …”

Synthesis of Li2MgIr and LiMgIrH6: Guidance from DFT

“…First, some of the hydrogen uptake can be attributed to the observed formation of the minor phase Mg 3 IrH 5 and to the production of LiH, as we propose below. Second, Ir is known to form (IrH 6 ) −3 complexes with hydrogen, as in Li 3 IrH 6 and Na 3 IrH 6 [11]. Such configurations obey the "18 electron rule" reflecting closedshell-type electronic stability.…”

“…On the right in Fig. 4 is a view of the (IrH 6 ) −3 anion complex highlighting the octahedral coordination of Ir by H. Not readily apparent is the fact that the Li 4e ions need to be eliminated from the parent structure in order to accommodate the six H atoms, and that elimination enables bond distances similar to those in other Ir hydride systems [11]. Given the LiMgIrH 6 hydride stoichiometry and the previous observation that the Li 4e ions are only loosely coordinated in the Li 2 MgIr parent lattice, and so are most likely prone to migration under appropriate conditions, we propose that the hydriding reaction is…”