Polycyclic aromatic hydrocarbons (PAHs) have been employed as models to investigate the functionalization of SWNTs sidewalls with transition metal complexes at the NL-DFT level, pointing out the most favorable coordination sites and the electronic properties of the resulting system. Molecular fragments topologically resembling the honeycomb lattice have been chosen, i.e., the pyrene (C16H10) and the coronene (C24H12) to respectively investigate an eta(2) interaction on the C-C bond and an eta(6) interaction on the hexagonal ring with suitable transition metal complexes. To reproduce the curvature of both (n,0) and (n,n) SWNTs surface, constrained geometry optimizations have been performed on these systems. Both geometrical parameters and bonding energies for the (PH3)(2)M(C16H10) (M = Ni, Pt) complexes suggest that eta(2) bonding of metal fragments to nanotubes is much weaker than to fullerene, making questionable the stability of eta(2) M(PH3)(2) complexes of carbon nanotubes. The eta(2) metal interaction with (9,0) and (5,5) bent-constrained pyrene models do not show any dependence on the (n,m) chiral vector. The analysis of the Cr(CO)(3)(C24H12) complexes shows that the (9,0) and (5,5) bent-constrained coronenes, modeling carbon nanotubes, enforce the chromium atom to arrange an eta(2) or eta(4) coordination on the hexagonal ring, rather than an eta(6) coordination, that is more favorable for the planar aromatic systems of benzene and planar coronene, modeling graphene sheets.