Metal–CO Bonding in Mononuclear Transition Metal Carbonyl Complexes

Abstract

DFT calculations have been carried out for coordinatively saturated neutral and charged carbonyl complexes [M(CO) n ] q where M is a metal atom of groups 2–10. The model compounds M(CO) 2 (M = Ca, Sr, Ba) and the experimentally observed [Ba(CO)] + were also studied. The bonding situation has been analyzed with a variety of charge and energy partitioning approaches. It is shown that the Dewar–Chatt–Duncanson model in terms of M ← CO σ-donation and M → CO π-backdonation is a valid approach to explain the M–CO bonds and the trend of the CO stretching frequencies. The carbonyl ligands of the neutral complexes carry a negative charge, and the polarity of the M–CO bonds increases for the less electronegative metals, which is particularly strong for the group 4 and group 2 atoms. The NBO method delivers an unrealistic charge distribution in the carbonyl complexes, while the AIM approach gives physically reasonable partial charges that are consistent with the EDA-NOCV calculations and with the trend of the C–O stretching frequencies. The AdNDP method provides delocalized MOs which are very useful models for the carbonyl complexes. Deep insight into the nature of the metal–CO bonds and quantitative information about the strength of the [M] ← (CO) 8 σ-donation and [M(d)] → (CO) 8 π-backdonation visualized by the deformation densities are provided by the EDA-NOCV method. The large polarity of the M–CO π orbitals toward the CO end in the alkaline earth octacarbonyls M(CO) 8 (M = Ca, Sr, Ba) leads to small values for the delocalization indices δ(M–C) and δ(M···O) and significant overlap between adjacent CO groups, but the origin of the charge migration and the associated red-shift of the C–O stretching frequencies is the [M(d)] → (CO) 8 π-backdonation. The heavier alkaline earth metals calcium, strontium and barium use their s/d valence orbitals for covalent bonding. They are therefore to be assigned to the transition metals.

Variational Energy Decomposition Analysis of Charge-Transfer Interactions between Metals and Ligands in Carbonyl Complexes

“…These electronic and energetic features are reflected by the overall and individual contributions in the stretch vibrational frequency of CO due to synergistic σand π-resonance. 24 Complementary to the recent study of a broad range of M− CO complexes by Frenking et al, 40 further showing the usefulness of the Dewar−Chatt−Duncanson model, the present work, which was originally submitted in January, 2021 and delayed because a distinguished referee considered it not inorganic enough, illustrated a well-defined procedure to quantify the structural, energetic, and spectroscopic origin of the CO ligand to metal σ-donation and metal to CO π-back conjugation. These analyses also provide a practical approach to understand the extent of non-innocence of metal−ligand interactions since both diabatic and adiabatically optimized structures featuring purely Coulomb interactions, only σdonating, and exclusively π-conjugation effects can be blocklocalized and variationally determined.…”

Bonding situation in isolable silver(I) carbonyl complexes of the Scorpionates

“…Carbon monoxide is one of the most important ligands in transition metal chemistry 1–8 . Several industrial processes such as Fischer‐Tropsch synthesis, water‐gas shift reaction, hydroformylation and acetic acid synthesis mediated by transition metals involving CO as a reagent are known 9–12 .…”

“…Such compounds have been termed “nonclassical” 6 or “mainly σ‐only” 52 carbonyls. Considering the importance of metal carbonyls, a number of groups, including Prof. Gernot Frenking particularly in recent years, 8,53–59 have investigated ways of explaining the nature of metal‐CO interaction in these two families of compounds and resulting effects, especially on the CO stretch of MCO groups 6,60–66 . These studies suggest that the effect of the charge on the polarization of the bonding orbitals of CO as a major factor in blue‐shifts 6,53 …”

“…39 The CO stretching frequency values of compounds B and C also demonstrate one of the most attractive features of the tris(pyrazolyl)borate ligand support (which belongs to a family of ligands known as Scorpionates)i.e., their steric and electronic tunability through the variations of substituents on the pyrazolyl moieties at the 3-, 4-, and 5-positions leading to notable effects on the metal fragment supported by these ligands. [46][47][48] The metalÀCO bonding interactions are typically discussed using the well-known DewarÀChattÀDuncanson (DCD) model 49,50 8,[53][54][55][56][57][58][59] have investigated ways of explaining the nature of metal-CO interaction in these two families of compounds and resulting effects, especially on the CO stretch of M CO groups. 6,[60][61][62][63][64][65][66] These studies suggest that the effect of the charge on the polarization of the bonding orbitals of CO as a major factor in blue-shifts.…”

Iron pentacarbonyl ligands on silver scorpionates

“…In addition, the [Ag( i )] ← M(CO) 5 donor–acceptor interaction (Δ E orb(2) ) becomes stronger when going down in the group 8 (Fe < Ru < Os), which agrees with the higher donor nature of the heavier M(CO) 5 fragments as compared to the parent Fe(CO) 5 . 20 As a consequence of these enhanced electrostatic and orbital interactions, the computed total interaction energy between the LAg( i ) and M(CO) 5 fragments is stronger than that in the parent silver-iron complex 3 , which is reflected in a higher blue-shift of the corresponding ν (CO) stretching frequencies (see Table 1).…”