The co-ordination and cleavage reactions of aluminium borohydride with ligands containing elements from Groups V and VI

Abstract

Aluminium borohydride reacts with triphenylphosphine to form initially a 1 : 1 adduct; subsequent addition of excess of ligand causes decomposition of this adduct, and results in cleavage of the AlH₂B bridges. With ammonia the initial reaction product is a 1 : 2 adduct, Al(BH₄)₃,2NH₃, but here excess of ligand cleaves the AlH₂B bridges unsymmetrically, and the final product is best formulated as [Al(NH₃)₆]³⁺(BH₄^–)₃. The relative strengths of the co-ordinate bond in a series of 1 : 1 adducts have been studied by displacement reactions which show NMe₃∼ PMe₃ > AsMe₃ and Me₂O > Me₂S. The similarity in the nitrogen and phosphorus compounds is discussed with relevance to the formation of 1 : 2 adducts in these cases.