Bonding in Binuclear Carbonyl Complexes M₂(CO)₉ (M = Fe, Ru, Os)

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Quantum Chemistry

Background:

• Metal carbonyl complexes are crucial in catalysis and materials science.
• Understanding the structural and bonding nuances of dinuclear metal carbonyls is essential for predicting their reactivity.
• Fe2(CO)9, Ru2(CO)9, and Os2(CO)9 represent a series where subtle electronic and structural changes occur down the group.

Purpose of the Study:

• To computationally investigate the structural preferences and bonding characteristics of M2(CO)9 (M = Fe, Ru, Os).
• To analyze the thermodynamic stability of these dinuclear complexes and their potential transformation into cyclic trinuclear carbonyls (M3(CO)12).
• To elucidate the nature of metal-ligand bonding, including sigma donation and pi back-donation, in bridging and terminal carbonyl ligands.

Main Methods:

• Quantum-chemical density functional theory (DFT) calculations using the BP86 functional.
• Inclusion of Grimme's D3(BJ) dispersion correction and a triple-ζ basis set.
• Analysis of bonding nature via Quantum Theory of Atoms in Molecules (QTAIM), Natural Bond Orbital (NBO) methods, and Energy Decomposition Analysis with Natural Orbital for Chemical Valence (EDA-NOCV).

Main Results:

• The energetically lowest structure for Fe2(CO)9 is a triply bridged D3h form, while Ru2(CO)9 and Os2(CO)9 favor singly bridged C2 structures.
• Calculated reaction energies indicate that Fe2(CO)9 is thermodynamically stable for forming M3(CO)12, but Ru2(CO)9 and Os2(CO)9 are not.
• Metal-CO bonds to bridging carbonyls are stronger than those to terminal carbonyls, with stronger sigma donation and pi back-donation for bridging ligands.
• EDA-NOCV analysis shows that M→CO π back-donation consistently exceeds M←CO σ donation.

Conclusions:

• The structural diversity and stability trends in M2(CO)9 complexes arise from a complex interplay of bonding forces and reorganization energies.
• Bridging carbonyl ligands are more strongly bound due to enhanced orbital interactions, but their formation requires energy compensation.
• The observed structures and stabilities are rationalized by a delicate balance of electronic and steric factors, highlighting group trends in metal carbonyl chemistry.