Several transamination reactions of B2(NMe2)4 (1a) with secondary amines have led to mixed tetraaminodiborane(4) compounds B2(NMe2)4 ‐ n(NR2)n (2–4), and B2(NC5H10)4 (1d) has been characterized by an X‐ray structure analysis which reveals the presence of a rather long B–B bond (1.75 Å). However, tetraaminodiboranes(4) of type R2N(Me2N)B‐B(NMe2)NR2 are more readily accessible from LiNR2 and B2(NMe2)2Cl2. Similarly, amination of B2(NMe2)2Cl2 with N,N'‐dimethylethylenediamine (7) yields B‐[bis(dimethyl‐amino)boryl]‐N,N'‐dimethyl‐1,3,2‐diazaborolidine (8), while reactions with Li(Me)N–CH2–CH2–N(Me)Li (9) lead also to 2,3‐bis(dimethylamino)‐1,4‐dimethyl‐1,4,2,3‐diazadiborinane (10) as the kinetically controlled product. This is further substantiated by the reaction of the B2(NMe2)2Br2 with 9 which gives exclusively the corresponding 1,4,2,3‐diazadiborinane 11. Diborane(4) dihalides B2(NMe2)2X2 (X = Cl, Br) react only in a 1:1 ratio with tmp‐B=N‐CMe3 (13) leading to 14a, b. However, both a 1:1 and a 1:2 methoxyboration of 13 has been observed with B2(OMe)4 with formation of 15 and 16.