Separating carbon dioxide (CO₂) from acetylene (C₂H₂) is one of the most critical and complex industrial separations due to similarities in physicochemical properties and molecular dimensions. Herein, we report a novel Ni-based three-dimensional framework {[Ni₄(μ₃-OH)₂(μ₂-OH₂)₂(1,4-ndc)₃](3H₂O)}_n (1,4-ndc = 1,4-naphthalenedicarboxylate) with a one-dimensional pore channel (3.05 × 3.57 Ų), that perfectly matches with the molecular size of CO₂ and C₂H₂. The dehydrated framework shows structural transformation, decorated with an unsaturated Ni(II) centre and pendant oxygen atoms. The dynamic nature of the framework is evident by displaying a multistep gate opening type CO₂ adsorption at 195, 273, and 298 K, but not for C₂H₂. The real time breakthrough gas separation experiments reveal a rarely attempted inverse CO₂ selectivity over C₂H₂, attributed to open metal sites with a perfect pore aperture. This is supported by crystallographic analysis, in situ spectroscopic inspection, and selectivity approximations. In situ DRIFTS measurements and DFT-based theoretical calculations confirm CO₂ binding sites are coordinatively unsaturated Ni(II) and carboxylate oxygen atoms, and highlight the influence of multiple adsorption sites.