A flexible and porous metal organic framework, based on Co-II connectors and benzotriazolide-5-carboxylato linkers, is shown to selectively react with guest molecules trapped in the channels during the sample preparation or after an exchange process. Stimulated by a small crystal shrinking, upon compression or cooling, the system undergoes a reversible, nonoxidative nudeophilic addition of the guest molecules to the metal ion. With dimethylformamide, only part of the penta-coordinated Co atoms transform into hexa-coordinated, whereas with the smaller methanol all of them stepwise increase their coordination, preserving the crystallinity of the solid at all stages. This extraordinary example of chemisorption has enormous implications for catalysis, storage, or selective sieving.