A. series of iridium complexes Containing phenanthro [4,5-abc] phenazino [11,12-4]iinidazoh-2-ylidene and acetonaphtho[1,2-b]quinoxaline[11,12-d]imidato1-2-ylidene ligands have been obtained and fay, charatterized. These complexes display highly extendech, polyaromatic systems attached to the backbone of the I\lheterocyClic carbene. The presence of this extended, polyaromatic system makes the electron donating character Of these ligands sensitive to the presence of pi-stacking additives, such as pyrene and hexafluorobentene. The computational Studies predict that the addition of pyrene affords an increase of the electron donating character Of the polyaromatit ligand (Tolman electronic parameter (TEP) decreases), while the.addition, of hexaflubroberizerie has the opposite effect (TEP increases). This prediction is experimentally corroborated by IR Spectroscopy, by measuring the shift of the CO stretching bands of a series of-IrCl(NHC)(CO)(2) complexes, where NHC,is the N-hetetocyclic carbene ligand with the polyaromatic systern. Finally, the energy of the pi-stacking,interaction Of one of the key Ir(I) complexes with pyrene and hocafluortibenzene has been estimated by using the Benesi-Hildebrand treatment, based on the delta-shifts observed by H-1 NMR spectroscopy.