The functionalization of single-walled carbon nanotubes (CNTs) with Vaska's complex trans-Ir(CO) Br(PPh3)(2) has been investigated by means of hybrid quantum mechanics/molecular mechanics (QM/MM) calculations. The formation of a stable adduct has been experimentally evidenced by Wong et al. ( Nano Lett. 2002, 2, 49), but microscopical details on the metal-nanotube interaction are still unclear. Our calculations show a low propensity to eta(2) coordination of Vaska's complex with the perfect hexagonal network of CNTs. Rather, a stronger interaction takes place when the transition metal center coordinates to carbon atoms belonging to pentagonal rings, as in topological defects or end-caps.