The olefin complexes Cp-2*M(CH2=CH(R))(H) (R = H (1), CH3 (2); M = Nb (a), Ta (b)) react cleanly with catecholborane (HBCat) and HBO2C6H3-4-'Bu (HBCat') to give Cp-2*M(H(2)BCat) (M = Nb (5a), Ta (5b)) and Cp-2*M(H(2)BCat') (M = Nb (6a), Ta (6b)) and the anti-Markovnikov hydroboration products CatBCH(2)CH(2)R and Cat'BCH2CH2R. Compounds 2a and 2b react with DBCat' (DBO2C6H3-4-'Bu) to afford the deuterated analogs Cp-2*M(D(2)BCat') (M = Nb (6a), Ta (6b)) where the deuterium label is incorporated exclusively in the metal complex. The hydride resonances in 6a exhibit large perturbations in chemical shift when deuterium is incorporated. On the basis of this isotopic labeling experiment, a mechanism is proposed where HBCat reacts with the 16-electron alkyl intermediates, Cp-2*MCH2CH2R (R = H (3), Me (4)), via sigma-bond metathesis or oxidative-addition/reductive-elimination sequences, to generate the alkylboranes and an intermediate hydride, Cp-2*MH, that is trapped by additional borane to give Cp-2*M(H(2)BCat) (5a and 5b). The solid-state structures for Cp-2*Nb(eta(2)-H2BO2C6H3-3-'Bu) (16) and Cp-2*Nb(eta(2)-BH4) (17) were determined by X-ray diffraction. The metal-boran distances in these two compounds are identical within experimental error. While related group 5 catecholateboryl compounds have pronounced boryl character, the structural parameters for the hydride and boryl ligands in 16 are consistent with formulation as either a borohydride complex or a borane adduct of ''Cp-2*NbH''. In contrast to other group 5 boryl complexes, 6a reacts readily with various two-electron ligands with elimination of HBCat'. For example, H-2 reacts reversibly to form Cp-2*NbH3 and HBCat', while ''BH3'' and CO react irreversibly to yield Cp*Nb(BH4) and Cp-2*Nb(H)(CO) with elimination of HBCat', respectively. Ethylene and propylene react at 40 degrees C to regenerate 1a and 2a, with elimination of HBCat'. When excess olefin is present, the liberated borane is converted to CatBCH(2)CH(2)R. Solutions of 1a and 2a catalyze olefin hydroboration under mild conditions. Relationships between the reactivity of 1a and 2a and other early metal and lanthanide catalysts are discussed.