The reaction of metalloxanes [{(LM)-L-Me-(OH)}(2)(mu-O)] [M = Al (1), Ga (2); L-Me = CH{CMe-(NAr)}(2)(-), Ar = 2,4,6-Me3C6H2, Me = methyl] with an excess of BH3 center dot D (D = tetrahydrofuran (THF), SMe2) affords annular metallaborate systems achieved through M-O-M cleavage. Compound 1 led exclusively to the formation of eight-membered ring systems [{(LAl)-L-Me(mu-O){B((OBu)-Bu-n)}(mu-O)}(2)] (3) and [{(LAl)-L-Me(mu-O)(BH)(mu-O)}(2)] (6), while for 2 the unprecedented six-membered ring systems [{((LGa)-L-Me)(2)(mu-O)}(mu-O)(2){B((OBu)-Bu-n)}] (4) and [((LGa)-L-Me)(2)(mu-O)(2){(B((OBu)-Bu-n)(2)(mu-O)}] (5) were observed. The use of BH3. THF with 1 and 2 led to the concomitant THF ring-opening reaction, while with BH3 center dot SMe2 in THF no such reaction observed. The metallaborates [(LAl)-L-Me{OB(pinacol)}(OH)] (7) and [{(LGa)-L-Me(OB(pinacol))}(2)(mu-O)] (8) were synthesized from pinacolborane and the corresponding metalloxane, providing structural evidence that supports the reaction pathways proposed for the formation of 3-6. Density functional theory studies were performed on 3-5 to assess the effect of the metal exchange between aluminum and gallium atoms on the energy of the general ring structures.