Compounds of the general formula t BuN(H)-SiMe 2 -N( t Bu)ElX 2 (El = B, X = Me (1m), F (1f), Cl (1c); El = AI, X = Me (2m), Cl (2c), Br (2b); El = Ga, X = Me (3m), Cl (3c), Br (3b); El = In, X = Me (4m), Cl (4c), Br (4b); El = Tl, X = Me (5m)) and t BuN(D)-SiMe 2 -N( t Bu)ElMe 2 (El = Al (2d), Ga (3d), In (4d)) have been characterized by IR and temperature dependent 1 H NMR techniques with respect to their structural changes in solution. A reversible, dynamic, intramolecular equilibrium between two enantiomeric four-membered rings has been established to be responsible for the changes in the spectra. The cyclic form B1, which arises from Lewis acid base interaction of the tert-butyl-amino with the ElX 2 group, transforms to the mirror-image B2 via an acyclic intermediate, in which reorientation of the groups by rotation or inversion can take place. In the boron derivatives (1m, 1c, and 1f) no comparable N-B interaction can be detected. The Lewis acidity of the ElX 2 group deduced from NH wave numbers increases within the following series: TlMe 2 <GaMe 2 ≈InMe 2 <AlMe 2 <GaBr 2 ≈InBr 2 <GaCl 2 ≈AlBr 2 ≈InCl 2 <AlCl 2 . Intramolecular hydrogen transfer from one nitrogen atom to the other can be excluded by comparison of the rate constant k ʜ vs. k ᴅ . The free enthalpy of activation Δ G* depends on the acidity of E1X 2 , the steric requirements and the relative masses of X. The syntheses and characterisation of 1f, 2b, 3b, 4b, 2d, 3d, and 4d are described.