Hydroboration of phenyl- and mesitylethyne with in situ formed HBCl 2 leads to an isomeric mixture of 1,1-bis(dichloroboryl)-2-phenylethane (4a) and l,l-bis(dichloroboryl)-1-phenylethane (4b) (ratio 3 : 1) and regioselectively to 1,1-bis(dichloroboryl)-2-mesitylethane (4c). Treatment of 4a/b and 4c with BI 3 gives the corresponding tetraiodo compounds 5, which undergo redox reactions with butyne-2 to form the 1,3-diiodo-4,5-dimethyl-2,3-dihydro-1,3- diborole derivatives 6a/b and 6c. Reactions of the diiodo-1,3-diboroles with AlMe 3 or LiMe result in the formation of the very air-sensitive pentaorganyl-1,3-diboroles 1a, 1c and the hexaorganyl-1,3-diborole 1b. The mixture of 1a/b as well as 1c react with bis(η 3 -allyl)nickel to give black products, which are chromatographed on silica with hexane as eluent. The brown solids were recrystallized from pentane to give the light-brown crystals of μ-η 5 ,η 5 -[2-benzyl-l,3 ,4 ,5 -tetramethyl-2 ,3-dihydrol, 3-diborolyl](η 3 -allyl)(η 4 -1,5-hexadiene)dinickel (2a, 34%) and μ-η 5 ,η 5 -[2-(2 ,4 ,6-trimethylbenzyl)- 1,3 ,4 ,5-tetramethyl-2 ,3-dihydro-1,3-diborolyl](η 3 -allyl)(η 4 -1,5-hexadiene)dinickel (2c, 43%). lb does not react with Ni(C 3 H 5 ) 2 to form any tripledecker. The compositions of the new compounds follow from spectroscopic and analytical data, and from X-ray structure analyses of 6a and of 2a. The structure of the η 4 -1,5-hexadiene-η 3 -allyl-dinickel triple-decker 2a finally disproved the alternative tris(allyl)dinickel arrangement.