Hydrogen storage capacity of Mg-decorated closo-hexaborate B6H62- has been studied using density functional theory. The binding strength of Mg atom is sufficiently large to ensure the stability of MgB6H62-. Each Mg atom can adsorb six H2 molecules. Moreover, the larger dipole moment combined with enhanced electrostatic field around the Mg atom originates from the charge transfer from B6H62- to Mg, accounting for the higher adsorption capacity of MgB6H62- than that of MgB6H6. Hydrogen storage capacity of MgB6H62- can be up to 11.1 wt% with an average binding energy between 0.23 eV and 0.34 eV. The electrostatic field around the Mg atom can be enhanced by controlling the charge state of the metal-organic complex, thereby significantly improving the hydrogen adsorption capacity.