The silica-supported transition metal hydrides (=Si-O-Si=)(=Si-O-)2Ta-H and (=Si-O-)xM-H (M, chromium or tungsten) catalyze the metathesis reaction of linear or branched alkanes into the next higher and lower alkanes at moderate temperature (25degrees to 200degreesC). With (=Si-O-Si=)(=Si-O-)2Ta-H, ethane was transformed at room temperature into an equimolar mixture of propane and methane. Higher and lower homologs were obtained from propane, butane, and pentane as well as from branched alkanes such as isobutane and isopentane. The mechanism of the step leading to carbon-carbon bond cleavage and formation likely involves a four-centered transition state between a tantalum-alkyl intermediate and a carbon-carbon final sigma-bond of a second molecule of alkane.

Reaction of Ta[CH2C(CH3)3]3[CHC(CH3)3] (1) with a silica dehydroxylated at 500 °C has been previously reported to produce a mixture of ⋮SiOTa[CH2C(CH3)3]2[CHC(CH3)3] (2a) and (⋮SiO)2Ta[CH2C(CH3)3][CHC(CH3)3] (2b). Treatment of these two surface organometallic complexes under 1 atm of hydrogen up to 200 °C leads to the formation of a surface tantalum(III) monohydride:  (⋮SiO)2TaIIIH (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 D2 (2 ± 0.2 mol equiv per tantalum); and 3 activates the C−H bond of cycloalkanes (C5 to C8) at room temperature to form the corresponding surface tantalum(III) monoalkyls (⋮SiO)2Ta-alkyl, with liberation of 1 ± 0.1 equiv of hydrogen. The surface tantalum-alkyls are transformed under oxygen into the corresponding (⋮SiO)2TaV(O)(O-alkyl). Quantitative determinations on the last three reactions are consistent with the formulation of 3 as a bis(siloxy)tantalum(III) monohydride (⋮SiO)2TaIIIH as the major surface species. Ta LIII-edge EXAFS studies of 3 confirm that two σ-bonded surface Si−O groups are attached to tantalum (Ta−O 1.89 Å). However, additional evidence is provided for a third surface oxygen (most probably siloxane) interacting weakly with the tantalum center (Ta···O 2.63 Å). A mechanism for the simultaneous formation of (⋮SiO)2TaIIIH 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.