Theoretical mechanistic calculations (DFT) on the reactivity of [{((Dip)Nacnac)Mg}2] ((Dip)Nacnac = [(DipNCMe)2CH](-), Dip = C6H3(i)Pr2-2,6) towards CO2 were carried out in order to rationalise the experimental formation of a carbonate (major product) and an oxalate (minor product). Despite its apparent similarity to f-element reactivity, the magnesium(I) bimetallic complex yields the carbonate through a concerted type of pathway rather than via a transient oxo-bridged intermediate. The latter is destabilised due to the electrostatic repulsion between the two magnesium centres. The small energy barrier difference between carbonate and oxalate formation (~10 kcal mol(-1)) may allow for the experimentally observed reactivity to be tuned by changing the sterics and/or electronic properties of the magnesium(I) complex.