Weak Interligand Interactions with Major Structural Consequences in Rh(CO)₂(CF₃COO)₃

A theoretical study was conducted to investigate the chemical nature of an unusual interaction observed between carbonyl and acetate ligands in the Rh(CO)2(CF3COO)3 complex. This interaction is intriguing because it is only nominally longer (0.1−0.2 Å) than a typical carbon−oxygen σ bond, yet is associated with only a modest (∼10 kcal/mol) energy lowering of the complex. A localized bonding molecular orbital that promotes the notion of charge sharing is present between the interacting ligands. Constrained geometry optimizations in tandem with Mulliken population analyses indicate that the interaction stems from the inability of Rh(III) with highly electron withdrawing ligands to back-donate properly into the carbonyl ligands. This produces a charge imbalance in the ligands, which sets the stage for nucleophilic attack by the acetate oxygen to the carbonyl carbon. This interaction causes a shift in the predicted values of both IR and 13C NMR signals, which are compared to experiment. For a full explanation of the 13C NMR shifts, two explicit solvent molecules were added to the model and found to induce interaction of both carbonyls with acetate ligands. The chosen density functional (B3LYP) and basis set were validated by comparing theoretically predicted structures and vibrational frequencies with experimentally determined values for several complexes.