In summary, this study reports a novel single-site cooperativity mechanism that enables high-capacity CO capture, representing a paradigm shift by moving beyond conventional dependence on long-range structural phase transitions. Unlike traditional cooperative systems, which incur substantial energy penalties during regeneration, this localized strategy achieves positive cooperativity through tandem spin transition and reversible migratory insertion at isolated Co(II)-methyl sites. This approach effectively resolves the long-standing trade-off between selectivity and regeneration energy. Effective implementation of this strategy requires integrating spin-active sites within a porous framework. However, several practical challenges remain to be addressed before practical application. For instance, the reactive cobalt-methyl bonds exhibit high sensitivity to atmospheric oxygen and moisture, and the approach also demands precise metal-ligand pairing. Despite these limitations, the mechanism of single-site cooperativity holds promise for extension to the capture or activation of other π-acidic gases, such as NO or SO₂, through deliberate engineering of the coordination environment.