The effect of ionization and CH3 ligand for hydrogen storage in Co‐ and Ni‐based organometallic compounds

Maximum capacities of the hydrogen storage in organometallic compounds consisting of Co and Ni atoms bound to C_mH_m ring (m = 4, 5; capped type) were, respectively, found as 3.48 and 3.49 wt % (Guo et al., Struct Chem, 2009, 20, 1107). Here, we extend this study to structures having a transition metal (TM) inserted in C_mH_m ring (inserted type), having TM located on either a C₄H or a C₅H molecule, and the CH₃ ligand bound to the organometallic compounds. We find that for the CoC₄H₄ and NiC₄H₄ complexes, the capped types are 1.39 eV and 1.41 eV higher in energy than the inserted types, respectively, while the ground states for CoC₅H₅ and NiC₅H₅ complexes are found to be the capped type, which are lower than the inserted types, respectively, by 1.27 eV and 1.31 eV. The maximum capacity of hydrogen storage reached 5.13 wt % for both of CoC₄H(H₂)₃ complex and the inserted‐type CoC₄H₄(H₂)₃ complex with a reasonable binding energy (0.3–1.0 eV per H₂). The positively charged C₄H₄ and C₅H₅ molecules do not only improve the capacity of hydrogen storage but also make all H₂ adsorbing in molecular form and keep the adsorption energy in an ideal range. After adding the CH₃ ligand to the compounds, the average adsorption energy of H₂ decreased to an ideal range 0.61–0.94 eV per H₂ and the stability of the compounds is also improved. Finally, we analyze the HOMO–LUMO gaps and display the kinetic stability when H₂ was added to organometallic compounds. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010