ReductiveCoupling of Carbon Monoxide by an AnionicCalcium Hydride: A Computational Mechanistic Study

We recently reported that a dimeric, anionic calcium hydride complex can perform the selective reduction and C–C coupling of carbon monoxide. Here, the mechanism of this reaction is investigated computationally. Stepwise coordination and reduction of CO is calculated, with the first molecule of CO being transformed into a Ca-bound formyl ligand. Subsequently, a second CO molecule coordinates to the same calcium center, and C–C bond formation proceeds via insertion of this second CO molecule into the Ca–C^formyl bond. This is in contrast to mechanisms reported for CO reduction with dimeric neutral and cationic Group 2 molecular hydrides, in which both Group 2 centers are involved in this key C–C bond-forming step. In a final step, the remaining hydride ligand located on the second calcium center is transferred to the newly formed CO-derived ligand, yielding a <i>cis</i>-ethenediolate unit, the single experimentally observed product. The <i>cis</i> selectivity can be explained by electrostatic repulsion in the pathway to the <i>trans</i> isomer. NBO/NLMO and energy decomposition analyses show that, in general, electrostatic interactions dominate the interaction between the CO-derived ligands and the calcium center.