Amine(NB)‐aminoborane internal complexes and 1,3,2‐diazaborolidines were recently identified as intermediates of the borane reduction of imidazolidino[1,2‐d]dithiazepines. Theoretical Mo calculations were used to obtain the structural, conformational, and electronic properties and heats of formation of the boron compounds in the framework of the semiempirical AM1 approximation. Internal constraints in the bicyclic system were analyzed through comparisons with the calculated molecular geometries, electron distribution of monocyclic, and open‐chain models. The geometry of the five‐membered diazaborolidine systems becomes distorted upon building up the complex bicyclic structure, compared to the nearly planar monocycles. The weak coordinative nitrogen–boron bond (calculated bond order around 0.5) in the internal complexes exerts less conformational strains in the adjoining nine‐membered ring containing the disulfide than does the covalent NB bond in the corresponding 1,2,3‐diazaborolidine. Thus, the internal constraints of the bicyclic system is taken up by the five‐membered ring to a lesser degree in the 1,3,2diazaborolidine than in the internal complex. © 1992 John Wiley & Sons, Inc.