[en] Reaction of MC1₄ (M = Zr, Hf, U, Th, Np) with LiBH₃CH₃ in chlorobenzene produces volatile, hexane-soluble M(BH₃CH₃)₄. Crystal structures are monomeric, tetrahedral species. Lewis base adducts prepared include U(BH₃CH₃)₄.THT, Th(BH₃CH₃)₄.L (L = THF (tetrahydrofuran), THT (tetrahydrothiophene), SMe₂, OMe₂), U(BH₃CH₃)₄.2L (L = THF, pyridine, NH₃), Th(BH₃CH₃)₄.2L (L = THF, THT, py, NH₃), M(BH₃CH₃)₄.L-L (M = U, Th; L-L = dme (1,2-dimethoxyethane), bmte (bis(1,2-methylthio)ethane), tmed (N,N,N',N'-tetramethylethylenediamine), dmpe (1,2-dimethylphosphinoethane)) and Th(BH₃CH₃)₄.1/2 OEt₂. Reaction of MC1₃ (M = Ho, Yb, Lu) with LiBH₃CH₃ in diethyl ether produces volatile, toluene-soluble M(BH₃CH₃)₃.OEt₂. Other Lewis base adducts prepared from M(BH₃CH₃)₃.OEt₂ include Ho(BH₃CH₃)₃.L (L = THT, THF, py), Ho(BH₃CH₃)₃.2L (L = THT, THF, py), Ho(BH₃CH₃)₃.tmed, Ho(BH₃CH₃)₃.3/2 L-L (L-L = dmpe, bmte), Yb(BH₃CH₃)₃.3/2 dmpe, Yb(BH₃Ch₃).L (L = THF, dme), Yb(BH₃CH₃)₃.2THF, and Lu(BH₃CH₃)₃.THF. By structural criteria, the bonding in actinide and lanthanide methyltrihydroborate complexes is primarily ionic in character even though they display covalent-like physical properties. Spectroscopic measurements indicate that there is some degree of covalent bonding in U(BH₃CH₃)₄