The reactivity of the two isoelectronic cations (Hg2+ and Tl3+) toward the cyclopropane ring is compared, and further evidence for the exclusive corner selectivity for Hg2+ is provided by isotope labeling. Cleavage of cyclopropyl derivative 1 with Hg(NO3)2, followed by KBr quenching, afforded the stable, rearranged organomercurial 3, whose transmetalation has been studied. Whereas reaction of 3 with Pd(II) afforded lactol 4, treatment with Me2CuLi resulted in the formation of cyclobutanol derivative (3 --> 29); analogous conjugate addition has also been accomplished (32 --> 35). Similarly, the organomercurial 22, obtained from 21 as the major product on the Hg(II)-mediated ring-opening, reacted with Me2CuLi or AlCl3 to give the ring-closure product 21. These reactions represent a novel method for the stereoselective construction of four- and three-membered rings. The stereochemistry of the key steps of these transformations has been established by using stereospecifically deuterated substrates 1b, 3b, 21b, and 22b.