Alkaline earth octacarbonyl complex
Alkaline earth octacarbonyl complexes are a class of neutral compounds that have the general formula M(CO)8 where M is a heavy Group 2 element (Ca, Sr, or Ba).
The metal center has a formal oxidation state of 0 and the complex has a high level of symmetry belonging to the cubic Oh point group.
The first reported alkaline earth octacarbonyl complex, Ba(CO)8, was first synthesized by Xuan Wu and Gernot Frenking in 2018.
[7] In the infrared spectrum for the octacarbonyl complex contains only one unique carbonyl stretching band suggesting these molecules have cubic Oh symmetry.
[9][10] Transition metal carbonyl complexes also depict a red-shifted absorption peak due to π back-donation interactions.
Computational methods of studying bonding interactions have produced varying results depending on the basis sets and reference states used.
Quantum chemical calculations using density functional theory confirm that Ca, Sr, and Ba can indeed utilize their (n-1)d in bonding to satisfy the 18-electron rule.
Computational methods such as QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis- Natural Orbitals of Chemical Valence) analyses as well as simple electron counting support a complex that abides by the 18-electron rule.
[1] Invoking this ligand-only orbital allows for satisfaction of the 18-electron rule in M(CO)8 complexes, and is stabilized by the field effect of the metal on the ligand cage.
[1][6] These electrons engage in strong π back-donation with the CO ligands and account for the red-shifted CO stretching frequency in the experimentally derived infrared spectra.
In this bonding model, the carbonyl-ligand anion cage ([(CO)8]−2) serves as a σ- and π-Lewis base and the metal center acts as a Lewis acid.
[3] Definitive proof of this bonding model would undermine the discovery of an alkaline earth complex that abides by the 18-electron rule.
Under this model, metal clusters treated as "giant atoms", and electron energy levels interacting with the spheroid charge distribution correspond to super shells where the resulting magic numbers are 2, 8,18, 20, 32, 40.
[17] A generic octacarbonyl complex adopts cubic Oh symmetry and can be viewed as a homogenous spherical field according to the Jellium model.
[14] Since full saturation of the occupied valence orbitals to form a closed shell species requires a total of 20 electrons, the magic number 20 is fulfilled.
