The reaction of [Ta(=CHtBu)(CH(2)tBu)(3)] or [CpTa(CH3)(4)] with a silica partially dehydroxylated at 700 degreesC gives the corresponding monosiloxy surface complexes [(=SIO)Ta(=CHtBu)(CH(2)tBu)(2)] and [(=SiO)Ta(CH3)(3)Cp] by eliminating a a-bonded ligand as the corresponding alkane (H-CH(2)tBu or H-CH3). EXAFS data show that an adjacent siloxane bridge of the surface plays the role of an extra surface ligand, which most likely stabilizes these complexes as in [(equivalent toSiO)Ta(=CHtBu)(CH(2)tBU)(2)(equivalent toSiOSiequivalent to)] (1a') and [(equivalent toSiO)Ta(CH3)(3)Cp*(equivalent toSiOSiequivalent to)] (2a'). In the case of [(=SiO)Ta(equivalent toCHtBu)(CH(2)tBu)(2)(equivalent toSiOSiequivalent to)], the structure is further stabilized by an additional interaction: a C-H agostic bond as evidenced by the small J coupling constant for the carbenic C-H (J(C-H) = 80 Hz), which was measured by J-resolved 2D solid-state NMR spectroscopy. The product selectivity in propane metathesis in the presence of [(=SIO)Ta(equivalent toCHtBu)(CH(2)tBu)(2)(equivalent toSiOSiequivalent to)] (1a') as a catalyst precursor and the inactivity of the surface complex [(equivalent toSIO)Ta(CH3)(3)Cp*(equivalent toSiOSiequivalent to)] (2a') show that the active site is required to be highly electrophilic and probably involves a metallacyclobutane intermediate.