f-Block chemistry offers an opportunity to test current knowledge of chemical reactivity. The energy dependence of lanthanide cation (Ln⁺ = Ce⁺, Pr⁺, Nd⁺–Eu⁺) and actinide cation (An⁺ = Th⁺, U⁺–Am⁺) oxidation reactions by CO₂, was observed by inductively coupled plasma tandem mass spectrometry. This reaction is commonly spin-unallowed because the neutral reactant (CO₂, ¹Σ⁺_g) and product (CO, ¹Σ⁺) require the metal and metal oxide cations to have the same spin state. Correlation of the promotion energy (E_p) to the first state with two free d-electrons with the reaction efficiency indicates that spin conservation is not a primary factor in the reaction rate. The E_p likely influences the reaction rate by partially setting the crossing between the ground and reactive states. Comparison of Ln⁺ and An⁺ congener reactivity indicates that the 5f-orbitals play a small role in the An⁺ reactions.