The preparation of tantalaziridine-hydride complex (ArrBuCH(2)]N)(2)(eta(2)-'Bu(H)CNAr)TaH (1) is reported (Ar = 3,5-Me2C6H3). While stable for months in the solid state at -35 degrees C, in solution this complex undergoes partial conversion to isomeric hydride (Ar[(BuCH2)-Bu-t]N)(2)(k(2)-CH2C(Me)(2)CH2NAr)TaH (2). Although 1 and 2 exist in equilibrium in benzene solution, complex 2 can be isolated cleanly from 1 by selective precipitation using cold n-pentane; solid-state structures for both 1 and 2 are presented. Exposure of 1 to ca. 1 atm of CO2 results in the production of methylene diolate complex {(Ar[(BuCH2N)-Bu-t)2(eta(2)-(t)B3u(H)CNAr)Ta}(2)(mu-OCH2O) as a mixture of rac and mesodiastereomers (3r,m). Similar reactivity for the Nb congener of 1 is reported herein. Furthermore, methylene diolate complex {(Ar[(BuCH2)-Bu-t]N)(2)(K2CH2C(Me)(2)CH2NAr)Ta}(2)(mu-OCH2O) (4) is produced from 2 upon treatment with CO2 and was characterized crystallographically. Complexes 3r,m (and the Nb analogues) as well as 4 establish the feasibility of the formation of methylene diolate products from CO2 and two terminal transition-metal hydrides. Reaction of formate (Ar[Bu-t]N)(3)TiOC(O)H with 1 generates the related, structurally characterized heterobinnetallic complex ((ArBuCH2)-Bu-t]N)(2)(eta(2)-Bu-t(H)CNAr)TaOCH2OTi(N[Bu-t]Ar)(3) (5), which further contributes to the class of complexes reported herein that effectively stabilizes an unusual H2CO22- ligand between two metal centers.