The treatment of trimethylaluminum with 9-hydroxy-9-borabicyclo[3.3.1]nonane 9-(OH)-9-BBN afforded a dimeric species [(mu-9-BBN-9-O)AlMe2](2) (1) in low yield and methylalumoxane (MeAlO)(n) as well as 9-Me-9-BBN as the major products. When the anhydride (9-BBN)(2)O was used instead of 9-(OH)-9-BBN die mixture of (MeAlO)(n) and 9-Me-9-BBN was formed exclusively. Contrary to this, reactions of (9-BBN)(2)O with trimethylgallium and trimethylindium proceeded with the cleavage of only one metal-carbon bond. The compounds [(mu-9-BBN-9-O)MMe2](2), where M = Ga (2), In (3), were isolated in high yields. Reactions of tetraethyldibroxane Et4B2O with trimethyl derivatives of aluminum, gallium, and indium were also studied. The structures of 1-3 were determined by X-ray diffraction showing a significant influence of the metal center on the pp(pi) interaction between boron and oxygen. The molecular structure of the similar dimer [(mu-9-BBN-9-O)GaBu2t](2) (3) is also reported.