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1,2-diphosphahexaborane(4)2343 Tetrabromodiborane(4), B2Br4, forms a 1: 2 adduct 1 with PBr3, phosphane or 1,2-bis(dichlorophosphanyl)ethane to give B2C1, . which reacts at 350°C to closo-1,2-PzB4Br4 (4a) in 25% yield. P(SiMe& (2) and B2C14 . ClzPCHzCHzPClz (3), respectively.Compound 4a neither rearranges to the 1,6-isomer nor decom-Compound 2 represents the first phosphanediborane(4) poses to an appreciable extent upon heating to 650°C. B2C14 monoadduct with two boron atoms in different coordination reacts with equimolar amounts of either tris(trimethylsily1)-environment.

The 12-vertex closo-phosphaborane 1,7-P2B10Cl10 (1) has been prepared in low yield from the pyrolysis reaction of B2Cl4 with PCl3 at temperatures above 400 degrees C. A single-crystal X-ray structure determination of 1 (monoclinic space group P2(1)/n with a = 9.239(2) A, b = 16.786(3) A, c = 15.739(3) a, beta = 93.25(3) degrees, and Z = 4) confirmed that, consistent with its 26 skeletal electron count, the phosphaborane adopts a distorted icosahedral structure with the phosphorus atoms in the 1,7-positions. Crystals of 1 contain toluene in a 1:1 molar ratio embedded between each P atom of neighboring cluster molecules. Alteration of the pyrolytic conditions resulted in the formation of the phosphaboranes P4B8Cl6 (2) and P2B8Cl8 (3), which were characterized spectroscopically. Copyrolysis of B2Cl4 with a mixture of PCl3 and AsCl3 at 450 degrees C generated the six-vertex arsaphosphaborane AsPB4Cl4 (4) and traces of the icosahedral arsaphosphaborane AsPB10Cl10. These compounds are examples of heteroboranes which contain two different group-15 atoms within a single molecule.