University of Michigan-Dearborn, Natural Sciences, Dearborn, MI 48128
Certain unsaturated metallocycles are thought to function as intermediates in acetylene metathesis reactions. It has been proposed that these intermediates achieve stability through electron delocalization and even aromaticity. However, these molecule have 4 electrons, a trade mark of antiaromaticity leading to extreme instability. Antiaromatic molecules are usually described as cyclic molecules with localized double bonds, low stability, and small anisotropy. Cyclobutadiene is a molecule most noted as the typical antiaromatic molecule. Substitution of one C atom with any period four atom leads to another antiaromatic molecule, for example silacyclobutadiene. However, substitution with a transition metal, as in the metallacyclobutadienes, not only removes the antiaromaticy but also creates a system with many aromatic traits. Although there is much contention about the definition of aromaticity and antiaromaticity, bond length equalization and magnetic properties are often used as the essential indication of aromaticity.
Metallacyclobutadienes are of interest to us because they have 4 pi electrons and yet appear to be aromatic, thus violating the Hückel 4n+2 rule. We present a study including the optimized geometries and vibrational frequencies of several isomers of Cl3MC3H3 where M is Hf, Ta, W, and Re using Density Functional Theory. In this work we present HOMO/LUMO orbitals, reaction energies, and magnetic character. NICS calculations show that the planar isomers of the metallacyclobutadienes to be aromatic in the pi region of the ring. Additionally, the presentation presents the possibility of antiaromaticity in metallacyclobutadienes.
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