In the presence of tmeda (N,N,N',N'-tetramethylethylenediamine), LiMe added to the iminoboranes RB.tplbond.NCMe3 (I; R = Me, Et, Bu) in a molar ratio of 2:1 to give cyclobutene-type azaboraazoniaborata ring compds. II. In solns. of II, the Li(tmeda) group undergoes a fluctuation from one N-atom to the other. The x-ray anal. shows that the Li(tmeda) group in cryst. II (R = Me) is bonded to one distinct N-atom; the 4 BN bond lengths in the nonplanar ring structure of II are found in a range between 142.5 and 163.3 pm; the structure is illustrated by the detn. of the R-R-deformation electron d. More stable iminoboranes I [R = CMe3, N(CMe3)SiMe3], are alkyllithiated by LiR1 (R1 = Me, Bu, CMe3) in a regio- and stereospecific way to give the cryst. aminoboranes RBR1:N(CMe3)Li(tmeda); a facile exchange of the Li(tmeda) group by electrophiles is obsd. (Me3C)2B:N(CMe3)Li(tmeda) crystallizes in the monoclinic system with a relatively short BN double bond of 138.3 pm.