Metal cluster topology. 21. Sigma aromaticity in triangular metal carbonyl clusters

Metal carbonyl triangles such as M3(CO)12 (M=Fe, Ru, Os) are normally assumed to have three two-center two-electron (2c–2e) bonds along the edges of their M3 triangles, which gives each of the three metal atoms the favored 18-electron rare gas configuration. However, this simple edge-localized bonding model does not account for the much greater stability of metal triangles relative to other metal polygons, e.g. the experimentally observed much greater stability of triangular Os3(CO)12 relative to square Os4(CO)16. An alternative model for bonding in M3 triangles uses the concept of σ-aromaticity first developed by Dewar and later Cremer and Schleyer to account for the stability and properties of cyclopropane derivatives. Applying this model to triangular metal carbonyls partitions the six orbitals and six electrons available for bonding within the M3 triangle into a core 3c–2e bond of Hückel topology formed by radial hybrid orbitals and a surface 3c–4e bond of Möbius topology formed by tangential p-orbitals. Similar core bonding is also postulated for the Pt3(CO)3(μ-CO)3 building blocks of platinum carbonyl structures including the [Pt(CO)2]n 2− stacks. However, the presence of the bridging CO groups in the Pt3(CO)3(μ-CO)3 units precludes the 3c–4e Möbius surface bonding postulated for M3(CO)12. This σ-aromaticity model for the chemical bonding in metal carbonyl triangles contains many of the features of the graph-theory derived model for the three-dimensional aromaticity in deltahedral boranes and related metal carbonyl clusters, particularly metal carbonyls containing octahedral M6 units.