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

Metal carbonyl triangles such as M-3(CO)(12) (M = Fe, Ru, Os) are normally assumed to have three two-center two-electron (2c-2e) bonds along the edges of their M-3 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 Os-3(CO)(12) relative to square Os-4(CO)(16). An alternative model for bonding in M-3 triangles uses the concept of sigma-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 M-3 triangle into a core 3c-2e bond of Huckel topology formed by radial hybrid orbitals and a surface 3c-4c bond of Mobius topology formed by tangential p-orbitals. Similar core bonding is also postulated for the Pt-3(CO)(3)(mu-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 Pt-3(CO)(3)(mu-CO)(3) units precludes the 3c-4e Mobius surface bonding postulated for M-3(CO)(12). This sigma-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 M-6 units. (C) 2003 Elsevier Science B.V. All rights reserved.