The global minimum of IrB12- is a C3v symmetric bowl-like structure in which the Ir atom is located on the concave side of the bowl, similar to its lighter congeners, CoB12- and RhB12- clusters. Although all these MB12- (M = Co, Rh, Ir) clusters show dynamical behaviour, analogous to that of the so-called ‘Wankel motors’, the energy barrier for the rotation of the inner B3 ring within the peripheral B9 ring is the lowest in the IrB12- case (5.0 kcal mol-1 only). The geometrical feature along with the lower interaction energy between B3 and MB9 moieties are responsible for a smaller rotational energy barrier in IrB12- than those in CoB12- and RhB12- clusters.