Phosphane-Tetrahalodiborane 4 adducts. Formation of closo-3,4,5,6-Tetrabromo-1,2-diphosphahexaborane 4

Tetrabromodiborane(4),B2Br4, forms a 1:2 adduct 1 with PBr3, which reacts at 350xC to closo-1,2-P2B4Br4 (4a) in 25% yield. Compound 4a neither rearranges to the 1,6-isomer nor decomposes to an appreciable extent upon heating to 650xC. B2Cl4 reacts with equimolar amounts of either tris(trimethylsilyl)-phosphane or 1,2-bis(dichlorophosphanyl)ethane to give B2Cl4 . P(SiMe3)3 (2) and B2Cl4 - Cl2PCH2CH2PCl2 (3), respectively. Compound 2 represents the first phosphane-diborane(4) monoadduct with two boron atoms in different coordination environment.