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1,3-diisopropyl-2,2-dimethyl-1,3,2,4?2-diazasilastannetidine (1b) and -plumbetidine (2b) are dimeric [(1b)2, (2b)2] in solution. At room temperature the structure of (1b)2 is fluxional. The dynamic behaviour is interpreted - on the basis of 1H, 13C, 29Si and 119Sn NMR data - as an intramolecular process in which the four-membered rings keep their identity. Such a process involves either concerted opening of the two coordinative Sn-N bonds and mutual slippage of the two rings, or consecutive cleavage of one of the coordinative Sn-N bonds and rotation about the other one. At room temperature the dimer (2b)2 is in equilibrium with its monomer 2b, whereas at low temperature the dynamic process corresponds to that established for (1b)2. In solutions which contain a mixture of the dimers (1b)2 and (2b)2, the presence of the mixed dimer 1b/2b can be proved unambiguously by consistent 29Si, 119Sn and 207Pb NMR data.