σ‐Aromaticity in Trinuclear and Tetranuclear Transition Metal Carbonyl Clusters: A Comprehensive Magnetic and Electronic Analysis

This study investigates the presence of σ ‐aromaticity in a series of trinuclear and tetranuclear metal carbonyl clusters with the general formulas [M 3 (CO) 12 ], [M 4 (CO) 16 ], [M 4 (CO) 15 ], and [M 4 (CO) 14 ], (where M = Fe, Ru, and Os), and isolobal trinuclear clusters [M 3 (CO) 12 ] 3– (where M = Mn, Tc, Re), and [M 3 (CO) 12 ] 3+ (where M = Co, Rh, Ir). Density functional theory was employed for all the geometry optimisation. Aromaticity was assessed using magnetic criteria, specifically nucleus‐independent chemical shifts (NICS) and gauge‐including magnetic‐induced ring currents (GIMIC). Our findings indicate that all cluster types exhibit σ‐aromaticity. Among all the clusters studied, the [Fe 3 (CO) 12 ] cluster displayed exceptionally strong σ‐aromaticity, evidenced by highly negative NICS values and a significant diatropic current, as confirmed by GIMIC analysis. Further analysis, utilising NICS‐scan, and electron density of delocalised bonds, confirmed the presence of pronounced σ‐delocalisation in these metal carbonyl clusters.