Aromaticity of tri- and tetranuclear metal-carbonyl clusters based on magnetic criteria

Recently, the sigma-aromaticity model proposed for cyclopropane by Dewar was employed to account for the stability of Group 8 trinuclear metal-carbonyl compounds [M-3(CO)(12)] (M = Fe, Ru, Os). This paper further examines this hypothesis and provides the first quantitative evidence for the (sigma-aromatic/antiaromatic nature of the [M-3(CO)(12)]/[M-4(CO)(16)] species based on structural and nucleus-independent chemical-shift analysis. In addition, the extent of electron delocalization in tetrahedral [M-4(CO)(14)] and butterfly [M-4(CO)(15)] is analyzed and compared to prototype cycloalkanes. While remarkable analogies exist between metal-carbonyls and cycloalkanes, transition metals provide additional overlap possibilities that affect both the ring strain and the magnetic properties of metal-carbonyl rings and cages.