The reaction dynamics of the boron monoxide radical ((BO)-B-11; X-2 Sigma(+)) with diacetylene (C4H2; X-1 Sigma(+)(g)) were investigated at a nominal collision energy of 17.5 kJ mol(-1) employing the crossed molecular beam technique and supported by ab initio and statistical (RRKM) calculations. The reaction is governed by indirect (complex forming) scattering dynamics with the boron monoxide radical adding with its boron atom to the carbon-carbon triple bond of the diacetylene molecule at one of the terminal carbon atoms without entrance barrier. This leads to a doublet radical intermediate ((C4H2BO)-B-11), which undergoes unimolecular decomposition through hydrogen atom emission from the Cl carbon atom via a tight exit transition state located about 18 kJ mol(-1) above the separated products. This process forms the hitherto elusive boronyldiacetylene molecule ((HCCCCBO)-B-11; X-1 Sigma(+)) in a bimolecular gas phase reaction under single collision conditions. The overall reaction was determined to be exoergic by 62 kJ mol(-1). The reaction dynamics are compared to the isoelectronic diacetylene (C4H2; X-1 Sigma(+)(g))-cyano radical (CN; X-2 Sigma(+)) system studied previously in our group. The characteristics of boronyldiacetylene and the boronyldiacetylene molecule ((HCCCCBO)-B-11; X-1 Sigma(+)) as well as numerous intermediates are reported for the first time.