This chapter discusses the CO 2 molecule in its ground and excited states, correlating the energy to the molecular geometry. The effect of adding or taking out an electron is illustrated, opening the way to the coordination of CO 2 to metal centers. Several modes of bonding of CO 2 are presented and the IR and multinuclear NMR spectroscopic data of transition metal complexes or adducts with Lewis acids and bases are commented. The reactivity of the coordinated heterocumulene is presented through several examples. The use of IR and NMR techniques for determining the molecular behavior of transition metal complexes in solution is exemplified.