Reaction of monoborane with tris(cyclopentadienyl)uranium(IV) derivatives

The compounds [UX(cp)3] (cp = ?5- cyclopentadienyl; X = Me, Et, Ph, NEt2, OMe, F, CONEt2, or COBu) react in toluene at room temperature with the labile adducts BH 3·L (L = tetrahydrofuran, BH3, or Me2S) giving the well known product [U(BH4)(cp)3], whereas [UX(cp)3] (X = Cl, Br, or I) seem to be unreactive even at high temperature. In some cases (X = Me, Et, or Ph), intermediate species such as [U (BH3X)(cp)3] were detected. A general reaction mechanism involving a hydride-bridged intermediate is proposed.