Although CO insertion is a predominant phenomenon in diamine-functionalized Mg (dobpdc) (dobpdc =4,4-dioxidobiphenyl-3,3'-dicarboxylate) adsorbents, a high-performance metal-organic framework for capturing CO , the fundamental function of the diamine carbon chain length in the mechanism remains unclear. Here, Mg (dobpdc) systems with open metal sites grafted by primary diamines NH -(CH ) -NH were developed, with en (n=2), pn (n=3), bn (n=4), pen (n=5), hn (n=6), and on (n=8). Based on CO adsorption and IR results, CO insertion is involved in frameworks with n=2 and 3 but not in systems with n≥5. According to NMR data, bn-appended Mg (dobpdc) exhibited three different chemical environments of carbamate units, attributed to different relative conformations of carbon chains upon CO insertion, as validated by first-principles density functional theory (DFT) calculations. For 1-hn and 1-on, DFT calculations indicated that diamine inter-coordinated open metal sites in adjacent chains bridged by carboxylates and phenoxides of dobpdc . Computed CO binding enthalpies for CO insertion (-27.8 kJ mol for 1-hn and -20.2 kJ mol for 1-on) were comparable to those for CO physisorption (-19.3 kJ mol for 1-hn and -20.8 kJ mol for 1-on). This suggests that CO insertion is likely to compete with CO physisorption on diamines of the framework when n≥5.