Are Metallacyclopentadienes Always Non-Aromatic?

Even though metallacyclopentadienes (MCPs) are among the most common metallacycles, their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current density) aromaticity indices. The indices agree that metallacyclopentadienes are, in general, weakly aromatic at most. The 18e− complexes showed the expected weak aromaticity, and only the d8 molecules are somewhat anti-aromatic. However, the theoretical account of the aromaticity of the 16e− MCPs is more convoluted. We find that the aromatic criteria for a 16e−d4 ruthenacyclopentadiene disagree. The lack of agreement shows that significant electron delocalization is not always related to great stability or to strong diatropic currents.