Abstract Several transamination reactions of B 2 (NMe 2) 4 (1a) with secondary amines have led to mixed tetraaminodiborane (4) compounds B 2 (NMe 2) 4-n (NR 2) n (2–4), and B 2 (NC 5 H 10) 4 (1d) has been characterized by an X-ray structure analysis which reveals the