The Fischer-Tropsch reaction is initiated by direct CO dissociation for Iron catalyst even though a H-assisted mechanism may be easier on other metals. In the gas phase, the CO dissociation is only favorable for Fe-clusters composed by more than 11 atoms. We show here the remarkable effect of the support TiO2(110), making this dissociation exothermic for Fe4 and Fe5 clusters. The main factor for the CO activation is the electron transfer to the reducible support. The role of the TiO2(110) support is to transform the neutral cluster into a positively charged one for which CO dissociation is easier.