The incorporation of N,N'-dimethylethylenediamine into an expanded MOF-74 framework has yielded a material (mmen-Mg-2(dobpdc)) exhibiting "step-shaped" CO, adsorption isotherms. The coordination of mmen at the Mg open metal center is essential for the unique cooperative adsorption mechanism elucidated for this material. Despite its importance for carbon capture, there is as yet no experimental structure determination available for the underlying metal organic framework Mg-2(dobpdc). Our Mg-25 solid-state NMR data unravel the local Mg environments in several Mg2(dobpdc) samples, unambiguously confirming the formation of five coordinate Mg centers in the activated material and six-coordinate Mg centers in the solvent- or diamine-loaded samples, such as mmen-Mg2(dobpdc). A fraction of the Mg centers exhibit local disorder due to the framework deformation accompanied by the guest distributions and dynamics.