Magnetic and electrical resistivity properties of RMo8O14 (R = La, Ce, Pr, Nd, Sm) compounds containing different bicapped-octahedral Mo-8 clusters are discussed. Extended Huckel (EH) molecular calculations were carried out in order to study the influence of the position of metal capping atoms on the electronic structure of different Mo-8 isomers. Different optimal metal electron counts are possible for these clusters. Periodic density functional calculations confirm the molecular character of these compounds and allow the understanding of their semiconducting and magnetic properties.