The carboxamides RCONH2 (R = Ph, t-Bu) react with BEt3 and with bis(9H-9-borabicyclo[3.3.1]borane (I) to give RCONHBEt2 and II. RCONHBEt2 are largely monomeric (NMR) in soln. and dimeric as cryst. solids. II are assocd. and probably polymeric in soln. (NMR). The 1:1 adducts III (R = Ph, t-Bu) are formed on the reaction of RCONHBEt2 with I by borane exchange. The former is also obtained directly by treating RCONH2 with I. III (R = Ph) reacts at 140-150 DegC to give the air-stable heterocycle IV (X-ray structure).