A series of 4d/4f-polyarsenides, -polyarsines and -polystibines was obtained by reduction of the Mo-pnictide precursor complexes [{(CpMo)-Mo-t(CO)(2)}(2)(mu,eta(2:2)-E-2)] (E=As, Sb; Cp-t=tBu substituted cyclopentadienyl) with two different divalent samarocenes [Cp*Sm-2] and [(Cp-Me4nPr)(2)Sm]. For the reductive conversion of the Mo-stibide only one product was isolated, featuring a planar tetrastibacyclobutadiene moiety as an unprecedented ligand for organometallic compounds. For the corresponding Mo-arsenide a tetraarsacyclobutadiene and a second species with a side-on coordinated As-2(2-) anion was isolated. The latter can be considered as reaction intermediate for the formation of the tetraarsacyclobutadiene.