The exchange reactions of phosphorus dihalides RPX2 [R = iPr, cyclohexyl (Cy), tBu, (3,5-Me2-C6H3)CH2, Ph; X = Cl, Br], EtOPCl2, Et2NPCl2, Cl2PCH2PCl2, and Cl2PCH2CH2PCl2 with 2,2-dimethyl-4,5-[1,2-dicarba-closo-dodecaborano(12)]-1,3-diselena-2-silacyclopentane provide a straightforward route to 1,3,2-diselenaphospholanes. Some tellurium analogues were prepared from the dilithium salt [1,2-(LiTe)2-1,2-C2B10H10] and were more difficult to characterize. For the selenium compounds, some of the P-organosubstituted five-membered rings dimerize to give ten-membered rings, most readily with the more bulky alkyl groups such as tBu. This was not observed for the tellurium compounds. The conformations of the five-membered rings were studied by NMR spectroscopy and DFT methods. Striking structural differences were clear for the 2-phenyl-1,3,2-diselenaphospholane derivative in solution and in the solid state. All reactions were monitored by multinuclear magnetic resonance spectroscopy (1H, 11B, 13C, 31P, 77Se, and 125Te NMR), and solution-state structures could be proposed. Remarkably large secondary isotope effects as isotope-induced chemical shifts 2Δ12/13C(31P) exerted by the carborane carbon atoms were observed; this is apparently a special property of the five-membered ring. The gas-phase structures were optimized by DFT methods [B3LYP/6-311+G(d,p) level of theory], and the NMR parameters were calculated. Two 1,3,2-diselenaphospholanes and one dimer were characterized by X-ray analysis.