Hydrogenolysis of the dimethyl actinide metallocenes (C5Me 4SiMe3)2UMe2 and (C 5Me4H)2AnMe2 (An = Th, U) was examined for comparison with the hydrogenolysis of (C5Me 5)2AnMe2 that forms the hydrides [(C 5Me5)2ThH2]2, [(C 5Me5)2UH2]2, and [(C 5Me5)2UH]2. Parallel reactivity is not found with the (C5Me4SiMe3)- and (C5Me4H)- complexes. Instead, this study led to the first example of a "tuck-over" [μ-η5-C 5Me3H(CH2)-κC]2- dianion derived from (C5Me4H)- ligands by C-H bond activation and rare examples of a polymetallic thorium polyhydride compound and an organometallic Th3+ complex. (C5Me4SiMe 3)2UMe2 reacts with H2 to form the bis(tethered alkyl) complex (η5-C5Me 4SiMe2CH2-κC)2U, a product of C-H bond activation of the silylmethyl groups. The only identifiable product of hydrogenolysis of (C5Me4H)2UMe2 is (C5Me4H)3U. The first thorium complex of (C5Me4H)- was synthesized by reaction of 2 equiv of (C5Me4H)MgCl(THF) with ThBr4(THF) 4 to produce (C5Me4H)2ThBr 2. This complex reacts with MeLi to make (C5Me 4H)2ThMe2. The latter complex reacts with H2 to form the ligand redistribution product (C5Me 4H)3ThMe and the tetrametallic octahydride tuck-over complex (C5Me4H)4[μ-η5-C 5Me3H(CH2)-κC]2Th 4(μ-H)4(μ3-H)4. For comparison with the (C5Me4H)3U product, the thorium analogue, (C5Me4H)3Th, was synthesized by potassium reduction of a [(C5Me4H)3Th][BPh 4] intermediate obtained in situ from (C5Me 4H)3ThMe and [HNEt3][BPh4]. (C 5Me4H)3Th can also be prepared from KC 8 and (C5Me4H)3ThBr, obtained from KC5Me4H and ThBr4(THF)4.