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.