The anion [H2B2(mu-H)(mu-C4H8)(2)](-), II, reacts with zirconocene dichloride and with hafnocene dichloride to form (eta(5)-C5H5)(2)ZrCl{(mu-H)(2)BC4H8}, III, and (eta(5)-C5H5)(2)HfCl{(mu-H)(2)BC4H8}, IV, complexes, which contain a boracyclopentane ring that is bound to the metal through hydrogen bridges. In these reactions, 0.5 mel of the transannular hydrogen-bridged 10-membered ring compound B-2(mu-H)(2)(mu-C4H8)(2) is also formed. A possible reaction pathway for the formation of these complexes is proposed on the basis of time-lapse B-11 NMR spectroscopy. The methyl-substituted complex (eta(5)-C5H5)(2)Zr(CH3){(mu-H)(2)BC4H8}, V, is prepared from the reaction of III with CH3Li. The molecular structure of III was reported earlier. The structures of IV and V are reported here. Crystal data for IV: space group P2(1)/n (No. 14), a = 8.565(1) Angstrom, b = 14.786(3) Angstrom, c = 11.120(3) Angstrom, beta = 96.17(2)degrees, Z = 4. Crystal data for V: space group Pbca (No. 61), a = 8.436(1) Angstrom, b = 16.343(2) Angstrom, c = 20.827(2) Angstrom, Z = 8.