Aromaticity in metallabenzenes.

The electronic structure and bonding situation in 21 metallabenzenes (metal=Os, Ru, Ir, Rh, Pt, and Pd) were investigated at the DFT level (BP86/TZ2P) by using an energy decomposition analysis (EDA) of the interaction energy between various fragments. The aim of the work is to estimate the strength of the pi bonding and the aromatic character of the metallacyclic compounds. Analysis of the electronic structure shows that the metallacyclic moiety has five occupied pi orbitals, two with b1 symmetry and three with a2 symmetry, which describe the pi-bonding interactions. The metallabenzenes are thus 10 pi-electron systems. This holds for 16-electron and for 18-electron complexes. The pi bonding in the metallabenzenes results mainly from the b1 contribution, but the a2 contribution is not negligible. Comparison of the pi-bonding strength in the metallacyclic compounds with acylic reference molecules indicates that metallabenzenes should be considered as aromatic compounds whose extra stabilization due to aromatic conjugation is weaker than in benzene. The calculated aromatic stabilization energies (ASEs) are between 8.7 kcal mol(-1) for 13 and 37.6 kcal mol(-1) for 16 which is nearly as aromatic as benzene (ASE=42.5 kcal mol(-1)). The classical metallabenzene model compounds 1 and 4 exhibit intermediate aromaticity with ASE values of 33.4 and 17.6 kcal mol(-1). The greater stability of the 5d complexes compared with the 4d species appears not to be related to the strength of pi conjugation. From the data reported here there is no apparent trend or pattern which indicates a correlation between aromatic stabilization and particular ligands, metals, coordination numbers or charge. The lower metal-C5H5 binding energy of the 4d complexes correlates rather with weaker sigma-orbital interactions.