[en] The reaction of MCl₄ (M = Zr, Hf, U, Th, Np) with LiBH₃CH₃ in chlorobenzene produces volatile, hexane-soluble M(BH₃CH₃)₄. The U(IV) and Th(IV) methyltrihydroborates readily form Lewis base adducts. The adducts prepared are described in detail, and their dimerization is discussed. A steric model has been used to justify the stoichiometry and structures of M(BH₃CH₃)₄ and M(BH₄)₄ (M = U, Th) base adducts. The bidentate base adducts of U(BH₃CH₃)₄ show dynamic behavior. The pseudooctahedral species have two nonequivalent BH₃CH₃ sites in the solid state, two trans and two cis to the bidentate ligand. These two sites exchange rapidly at room temperature in solution but exchange can be slowed appreciably at lower temperatures so as to observe two nonequivalent BH₃CH₃ sites on the ¹H NMR time scale. A twisting mechanism has been proposed based on measured ΔG values for two site exchange. The U(III) species, U(BH₃CH₃)₃.2dmpe, has also been prepared by thermal decomposition of U(BH₃CH₃)₄.dmpe in toluene in the presence of excess dmpe. The reaction of MCl₃ (M = Ho, Yb, Lu) with LiBH₃CH₃ in diethyl ether produces volatile, toluene-soluble M(BH₃CH₃)₃.OEt₂. Lewis base adducts prepared from M(BH₃CH₃)₃.OEt₂ are discussed. By structural criteria, the bonding in actinide and lanthanide methyltrihydroborate complexes is primarily ionic in character even through they display covalent-like physical properties. Spectroscopic measurements indicate that there is some degree of covalent bonding in U(BH₃CH₃)₄