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Zusammenfassung:
We present ab-initio valence-bond calculations on free and coordinated CO2. By analogy to transition-metal complexes with coordinated CO2 three different coordination geometries for the CO2 molecule are considered: (a) pure carbon coordination, (b) pure oxygen coordination, (c) mixed carbon-oxygen coordination. It is shown that pure oxygen ((b) geometry) and mixed carbon-oxygen coordination ((c) geometry) are more favourable than pure carbon coordination. In all cases studied, the bonding between the CO2 moiety and the metal atom is described best as a CO2− anion interacting with a Ni cation. On the basis of the theoretical calculations, many observed and expected features (some already known and some predicted) of the interaction of CO2 and metal surfaces can be discussed. The electron transfer to the CO2 moiety drives the observed bent geometry of the coordinated CO2 molecule and is accompanied by an elongation of the C-O bond distance with respect to the free molecule. The bond elongation leads to a drastic lowering of the asymmetric C-O stretching frequency, and a change in the relative energy position of the photoelectron peaks. We also consider intermolecular interaction between the CO2− anion and surrounding neutral CO2 molecules via "solvation" in analogy to results of recent gas-phase cluster experiments. On the basis of the deduced metal-CO2 bonding scheme the reactivity of coordinated CO2 is investigated. Three reaction channels are considered: (a) dissociation into CO and O−, (b) oxidation to CO3− and CO32−, (c) disproportionation of CO2− + CO2 to CO3− and CO. On the basis of energetic considerations we argue that dissociation is likely to occur on transition-metal surfaces, while oxidation to carbonate species is more likely on noble metals due to the low binding energies for the dissociation products, namely oxygen and CO on these surfaces.
