The reaction of lanthanide and yttrium hydrocarbyls {Cp*2Ln(μ-Me)}2 (Cp* = tBuC5H4 (Cp‘), Ln = Y (1), Tb (2), Yb (3), Lu (4); Cp* = Me3SiC5H4 (Cp‘‘), Ln = Lu (5)), one of which, 2, has been characterized by X-ray crystal analysis, with various organosilicon, -germanium, and -tin hydrides (as well as some organoaluminum and -gallium hydrides) in hydrocarbon solution was found to yield the corresponding unsolvated dimeric lanthanide and yttrium hydrides {Cp*2Ln(μ-H)}2 rather than compounds with lanthanide−element (Si, Ge, Sn) bonding. Thus, the reaction involves hydride transfer to Ln rather than the silyl transfer studied earlier for pentamethylcyclopentadienyllanthanide hydrocarbyls. Dimeric compounds Cp*2Ln(μ-H)(μ-Me)LnCp*2 with different bridging ligands were isolated; they are intermediates in this reaction. Dimeric lanthanide and yttrium hydrides catalyze the H/D exchange in silanes. This catalytic reaction is most correctly described by a mechanism involving nucleophilic substitution at the silicon a...