The anion [H2B2(mu-H)(mu-C4H8)(2)](-) reacts with zirconocene dichloride or hafnocene dichloride to form (eta(5)-C5H5)(2)ZrCl{(mu-H)(2)BC4H8} or (eta(5)-C5H5)(2)HfCl{(mu-H)(2)BC4H8}, complexes which contain a borocyclopentane ring that is bound to the metal through hydrogen bridges. In these reactions the organodiborate anion disproportionates to form 1/2 mol of B-2(mu-H)(2)(mu-C4H8)(2) as well as the 5-membered borocyclopentane ring.