Theoretical Description of Hydride-hydride Interactions in Selected Hydrogen Storage Materials

In recent years there has been growing interest in the use of metal hydrides as hydrogen rich sources. The high content of hydride-hydride contacts H^δ-⋅⋅⋅^δ-H in these materials appears to be relevant for hydrogen formation. At present time there is no consensus whether these contacts are attractive or repulsive. Accordingly, the main goal of this article is to shed light on physical factors which constitute homopolar hydride-hydride interactions H^δ-⋅⋅⋅^δ-H in selected transition metal complexes i. e. HCoL₄, L=CO,PPh₃,PH₃. In order to achieve this goal, the charge and energy decomposition ETS-NOCV approach along with the Interacting Quantum Atoms (IQA) and reduced density gradient (NCI) are applied for the bonded adducts L₄CoH⋅⋅⋅HCoL₄. Based on DFT and correlated methods it has been shown, contrary to classical interpretations, that hydride-hydride interactions might be attractive and even far stronger than classical hydrogen bonds. The stability of the adducts is increased by phosphine ligand installation: overall H^δ-⋅⋅⋅^δ-H bonding energy changes in the order: CO≪PPh₃~PH₃. It has been revealed that depending on monomer's conformations H^δ-⋅⋅⋅^δ-H bonds are dominated by charge delocalization or London dispersion forces and the electrostatic term is also relevant. The side carbonyl ligands additionally stabilize the H^δ-⋅⋅⋅^δ-H bonded structures through covalent charge delocalizations and Coulombic contributors. Furthermore, the sterically crowded systems containing bulky phosphine ligands are supported by π⋅⋅⋅π stacking, C-H⋅⋅⋅π and C-H⋅⋅⋅H-Co. It is finally determined by IQA energy decomposition, that diatomic hydride-hydride interaction CoH⋅⋅⋅HCo is chameleon-like, namely, it is attractive in CO₄CoH⋅⋅⋅HCoCO₄ and (PH₃)₄CoH⋅⋅⋅HCo(PH₃)₄, whereas the repulsion is unveiled in (CO)₃(PPh₃)CoH⋅⋅⋅HCo(CO)₃(PPh₃) where the monomers are of C_s symmetry.