Aromaticity and Agostic Interactions as Stabilizing Factors in Trinuclear Rhenium Clusters with Low Electron Count
- 1 July 1989
- journal article
- research article
- Published by Taylor & Francis in Comments on Inorganic Chemistry
- Vol. 9 (1) , 37-59
Abstract
The uniquely featured overall Re═Re double bond structure in some classes of Re3 clusters is critically analyzed. Perturbation MO theory, fragment frontier orbitals and other qualitative concepts based on extended Hückel calculations are applied to the well known family of clusters whose precursor is the Re3Cl12 −3 cluster. Twelve electrons from three d4 Re(III) metals are populating σ and π⊥ MO levels whose distribution assumes the character of either σ and π⊥ aromaticity. In particular the π⊥ aromaticity of six electrons distributed over three rather than six atomic centers is attributable to the availability of metal dδ orbitals. Similarities and differences between the most classical aromatic system, i.e., benzene, are pointed out, π⊥ aromaticity is also the most stabilizing factor in the class of alkylated clusters derived from Re3Cl9 [e.g., Re3(CH3)9]. Here the metal electron deficiency [total electron count = 30] is partially overcome by agostic interactions between the metal atoms and the C─H bonds of the bridging CH3 groups which then become formal four electron donors. These latter interactions are proved to be at work by closely analyzing available structural data and by calculating their stabilizing effect at EHMO level. Finally, the reported compound Re3(PEt2Ph)2(CH3)6(μ-CH3)3 is critically analyzed and a possible alternative formulation is forwarded.Keywords
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