Structure and Neutral Homoaromaticity of Metallacyclopentene, -pentadiene, -pentyne, and -pentatriene: A Density Functional Study

Density functional calculations were carried out on a series of metallacycles ( 1 − 6 ) to analyze the bonding and specifically to find the presence of any metal−π interaction in them. While there is no interaction between the metal and the middle carbon atoms in metallacyclopentane ( 1 ) and metallacyclopentadiene ( 4 ), strong metal−π interaction is found in the other metallacycles. The metallacyclopentene ( 2 ) and metallacyclopentyne ( 5 ) are found to be neutral bishomoaromatic, while the metallacyclopentatriene ( 6 ) is neutral in-plane aromatic. The calculated nucleus-independent chemical shift values and other bonding parameters support the strong cyclic delocalization of electrons in 2, 5, and 6 . A comparison of the calculated hydrogenation energies of the parent hydrocarbons and the metallacycles indicates that the metal fragment nearly eliminates the strain energy.