Reaction of Ta[CH2C(CH(?)3)(3)](3)[=CHC(CH3)(3)](3) (1) With a silica dehydroxylated at 500 degrees C has been previously reported to produce a mixture of =SiOTa[CH2C(CH3)(3)](2) [=CHC(CH3)(3)] (2a) and (=Si0)(2)Ta[CH2C(CH3)(3)][=CHC(CH3)(3)] (2b). Treatment of these two surface organometallic complexes under 1 atm of hydrogen up to 200 degrees C leads to the formation of a surface tantalum(III) monohydride : (=SiO)(2)(TaH)-H-III (3) as a major product and surface Si-H groups. 3 has been characterized in the following way : it reversibly exchanges with deuterium to give the corresponding [Ta]-D species; 3 reacts with CH3I to give methane; 3 reacts quantitatively with di(tert-butyl)ketone to form the corresponding tantalum di(tert-butyl)methoxide; 3 reacts with D2O to give a mixture of HD and D-2 (2 +/- 0.2 mol equiv per tantalum); and 3 activates the C-H bond of cycloalkanes (C-5 to C-8) at room temperature to form the corresponding surface tantalum(III) monoalkyls (=SiO)(2)Ta-alkyl, with liberation of 1 +/- 0.1 equiv of hydrogen. The surface tantalum-alkyls are transformed under oxygen into the corresponding (=Si0)(2)Ta-V(=O)(O-alkyl). Quantitative determinations on the last three reactions are consistent with the formulation of 3 as a bis(siloxy)-tantalum(III) monohydride (=Si0)(2)(TaH)-H-III as the major surface species. Ta L(III)-edge EXAFS studies of 3 confirm that two sigma-bonded surface Si-O groups are attached to tantalum (Ta-O 1.89 Angstrom). However, additional evidence is provided for a third surface oxygen (most probably siloxane) interacting weakly with the tantalum center (Ta ... O 2.63 Angstrom). A mechanism for the simultaneous formation of (=SiO)(2)(TaH)-H-III 3 and Si-H groups from 2a is proposed, which involves a hydride transfer from tantalum to a neighbor silicon atom and the transfer of an oxygen from the same silicon atom to the more oxophilic tantalum.