The First Carbonyl Bond Dissociation Energies of M(CO)5 and M(CO)4(C2H2) (M = Fe, Ru, and Os):  The Role of the Acetylene Ligand from a Density Functional Perspective

Recent kinetics experiments have shown that the rate of carbonyl substitution in complexes of the type M(CO)4(C2R2), where M = Fe, Ru, and Os, is accelerated by factors of 102−1013 over their respective pentacarbonyl complexes. These substitution reactions have been shown to be dissociative in nature and show a marked metal dependence of the rate. The origin of the increased reactivity of these alkyne complexes was studied with nonlocal density functional theory (BLYP functional), using both effective core potential (ECP) and all-electron basis sets, in conjunction with Frenking's charge decomposition analysis (CDA) scheme and Bader's atoms in molecules (AIM) analysis. We found that the BLYP/ECP method predicted geometries very close to experiment for both the parent carbonyl and alkyne complexes (in this study C2H2 was used). The calculated CO bond dissociation energies (BDEs) were also found to agree well with experiment and mirrored the metal dependence trends observed experimentally for both M(CO)5 and M(CO)4(C2H2). By using the CDA scheme the nature of the acetylene ligand was characterized in both the reactant, M(CO)4(C2H2), and the unsaturated dissociation product, M(CO)3(C2H2): acetylene was found to act as a two-electron donor in the reactant complex (with only the π∥ orbitals of C2H2 actively donating to the metal) and as a four-electron donor (with both the π∥ and the π⊥ orbitals of C2H2 actively donating to the metal), increasing the stability of the otherwise 16-electron unsaturated dissociation product. The predicted structural changes along with the results of the AIM analysis fully support the CDA findings. At the BLYP/ECP level the observed rate enhancement compared to the parent M(CO)5 compounds was rationalized in terms of the metal dependence of the molecular orbital energy gap for the π⊥ C2H2→M interaction, in the unsaturated M(CO)3(C2H2) intermediate.