The two cluster anions, PtMgH₃⁻ and PtMgH₅⁻, were studied by photoelectron spectroscopy and theoretical calculations. Experimentally-determined electron affinity (EA) and vertical detachment energy (VDE) values were compared with those predicted by our computations; excellent agreement was found. The calculated structures of PtMgH₃⁻ and PtMgH₃ both exhibit η₂-bonded H₂ moieties. Activation of these H₂ moieties is implied by the elongation of their bond lengths relative to the bond length of free H₂. The calculated structures of PtMgH₅⁻ and PtMgH₅ both exhibit all-hydrogen, five-member, σ-aromatic rings. These attributes are responsible for this anion's special stability.