Electron delocalization in the metallabenzenes: A computational analysis of ring currents

Many metallabenzene complexes appear to exhibit an enhanced thermodynamic stability which has been attributed to the concept of aromaticity. Analysis of the ring currents induced by a magnetic field, either by direct visualization or by considering nuclear or nucleus-independent chemical shielding values (NMR or NICS), have become useful theoretical tools to characterize the aromaticity of many molecules involving the main group elements. We have analyzed 21 metallabenzenes using variations of these techniques, which take account of the large core and metal orbital contributions which often lead to transition-metal-containing systems exhibiting anomalous shielding values. Analysis of individual orbital contributions to both the ring currents and chemical shielding values based upon the ipsocentric and CSGT (continuous set of gauge transformations) methods has shown that complexes such as the 18 electron Ir or Rh(C5H5)(PH3)(2)Cl-2 molecules should be classed as aromatic, whereas the 16 electron complexes such as Os or Ru(C5H5)(PH3)(2)Cl-2 should not, despite having the same occupancy of pi-MOs. The differences can be directly attributed to the HOMO/LUMO b(2) in-plane (d(xy)) molecular orbital, which, when unoccupied, is available to disrupt the delocalized currents typical of aromatic systems. A range of Pd and Pt metallabenzenes with cyclopentadienyl and phosphine ligands is also discussed as having aromatic and nonaromatic character, respectively.