The recent theory-orientated discovery of record high-temperature superconductivity (T_{c}∼250 K) in sodalitelike clathrate LaH_{10} is an important advance toward room-temperature superconductors. Here, we identify an alternative clathrate structure in ternary Li_{2}MgH_{16} with a remarkably high estimated T_{c} of ∼473 K at 250 GPa, which may allow us to obtain room-temperature or even higher-temperature superconductivity. The ternary compound mimics a Li- or electron-doped binary hydride of MgH_{16}. The parent hydride contains H_{2} molecules and is not a good superconductor. The extra electrons introduced break up the H_{2} molecules, increasing the amount of atomic hydrogen compared with the parent hydride, which is necessary for stabilizing the clathrate structure or other high-T_{c} structures. Our results provide a viable strategy for tuning the superconductivity of hydrogen-rich hydrides by donating electrons to hydrides via metal doping. Our approach may pave the way for finding high-T_{c} superconductors in a variety of ternary or quaternary hydrides.