The Nature of Nonclassical Carbonyl Ligands Explained by Kohn–Sham Molecular Orbital Theory

Abstract

When carbonyl ligands coordinate to transition metals, their bond distance either increases (classical) or decreases (nonclassical) with respect to the bond length in the isolated CO molecule. C−O expansion can easily be understood by π‐back‐donation, which results in a population of the CO's π*‐antibonding orbital and hence a weakening of its bond. Nonclassical carbonyl ligands are less straightforward to explain, and their nature is still subject of an ongoing debate. In this work, we studied five isoelectronic octahedral complexes, namely Fe(CO)62+, Mn(CO)6+, Cr(CO)6, V(CO)6− and Ti(CO)62−, at the ZORA‐BLYP/TZ2P level of theory to explain this nonclassical behavior in the framework of Kohn–Sham molecular orbital theory. We show that there are two competing forces that affect the C−O bond length, namely electrostatic interactions (favoring C−O contraction) and π‐back‐donation (favoring C−O expansion). It is a balance between those two terms that determines whether the carbonyl is classical or nonclassical. By further decomposing the electrostatic interaction ΔVelstat into four fundamental terms, we are able to rationalize why ΔVelstat gives rise to the nonclassical behavior, leading to new insights into the driving forces behind C−O contraction.

Doping Platinum with Germanium: An Effective Way to Mitigate the CO Poisoning

“…Second, upon CO adsorption, the C−O stretching frequency is lowered (see Figure 2(e)). Such redshift originates from the Pt→CO π‐back‐donation to the 2π* MO, which results in a population of the CO antibonding MO, destabilizing the CO bond and lowering the C−O stretching frequency [51] . The effect of the red‐shift on the C−O frequency, however, is milder in the doped clusters, corresponding to a smaller π‐back‐donation.…”

Observation of inserted oxocarbonyl species in the tantalum cation-mediated activation of carbon dioxide dictated by two-state reactivity

“…This blue shift can be explained by negligible M → CO π back-donation originating from the high-valence oxidation state of the metal as demonstrated in "non-classical" transition metal carbonyls. [70][71][72][73][74][75] Moreover, the out-of-phase combination of asymmetric stretching motions of two CO 2 ligands is calculated to be 2466 cm −1 , while the in-phase asymmetric stretching combination has a vibrational frequency of 2489 cm −1 with weaker intensity. The second lowest-energy isomer IDb, which is higher in energy than IDa by 4.2 kcal mol −1 (Table 1), exhibits a nearly linear binding mode with the Ta⋯CO distances of 2.33 and 2.35 Å, respectively.…”

Understanding chemistry with thesymmetry‐decomposed Voronoideformation density charge analysis

“…In the first example, we use the symmetry‐decomposed VDD charge analysis and the EDA to study the nature of metal–CO coordination bonds in octahedral M(CO) 6 complexes with M = Fe 2+ , Mn + , Cr, V − , or Ti 2− 22 . As can be seen from Figure 5A, the variation of the metal center in these complexes influences the ligand CO equilibrium bond distance ( r CO ), which elongates varying the metal center from M = Fe 2+ ( r CO = 1.129 Å) to M = Ti 2− ( r CO = 1.190 Å).…”