Aromaticity of Some Metal Clusters: A Different View from Magnetic Ring Current

Area of Science:

• * Computational Chemistry
• * Quantum Chemistry
• * Materials Science

Background:

• * Aromaticity in small planar metallic clusters is traditionally associated with pi (π) electron delocalization.
• * Previous studies have not fully emphasized the contribution of sigma (σ) electrons to aromaticity and stability.
• * Understanding electron delocalization is key to predicting cluster stability and reactivity.

Purpose of the Study:

• * To re-evaluate the aromatic character of small planar metallic clusters focusing on σ electrons.
• * To investigate the role of σ electron delocalization in the thermodynamic stability of these clusters.
• * To apply magnetic ring current analysis as an indicator of aromaticity.

Main Methods:

• * Density Functional Theory (DFT) calculations using the B3LYP functional and 6-311G(d) basis set.
• * Construction of magnetic ring current maps to visualize electron delocalization.
• * Analysis of the symmetry of electronic excitations to confirm diatropic currents.

Main Results:

• * σ electron delocalization significantly contributes to the thermodynamic stability of metallic clusters.
• * The triatomic B₃⁺ core exhibits double σ and π aromaticity in complexes like [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺.
• * Planar pentacoordinated carbon clusters (e.g., C@Al₅⁺) are confirmed as σ aromatic, contrary to previous π aromatic assignments.
• * Mixed copper clusters (Cu₃Si₃⁺, Cu₃Ge₃⁺) and some beryllium hydride clusters show σ aromaticity.
• * Copper hydrides (Cu_nH_n) are found to be nonaromatic.
• * Specific Be₂-centered clusters exhibit σ aromaticity or double aromaticity.

Conclusions:

• * σ electrons are a critical, often overlooked, factor in determining the bonding characteristics and aromaticity of metallic clusters.
• * Magnetic ring current analysis is a valuable tool for assessing σ aromaticity.
• * The findings necessitate a revised understanding of aromaticity in inorganic clusters, incorporating σ electron contributions.