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tion. Complex 2 was prepared by the addition ofa suspension of (Et,NH)BPh, (75.9 nig, 0.18 mmol, from (Et,NH)CI and NaBPh,) in 5 mL of THF to a stirred orange-yellow solution of 1 (90 mg. 0.09 mmol) In 7 mL of THE The reaction was stirred overnight and the solvent was removed by rotary evaporation to leave a dark orange product. This was washed with 5 mL of toluene to remove oily by-products and left a 2: 1 mixture (by NMR spectroscopy of 2 and 3 (1 10 mg). Complex 2 was separated by recrystallization from THF: toluene (4:l).

The 1,2-dihaIogenodiborane(4) compound Br(Me,N)BB(NMe,)Br 1 reacts with Li(mes) (mes = C,H,- Me,4,6) or LiPh to form thederivatives Br(Me,N)BB(NMe,)(mes) 2, (mes)(Me,N)BB(NMe,)(mes) 3, and the previously reported (synthesised by a different route) species Ph(Me,N)BB(NMe,)Ph 4 in moderate yields. Compounds 2-4 are thermally stable, colourless, crystalline solids. All compounds were characterized by X-ray crystallography and N M R spectroscopy. Their structures have B-B bond lengths that vary from 1.682(16) A in 1 to 1.71 7(15) A in 3. In addition, all compounds display high angles (from 58.9 to 88.7') between the planes at the boron atoms and short B-N distances that are near 1.39 A. The structures of 1-4 represent, apparently, the first such acyclic examples for diborane(4) derivatives of the type B,X,(NR,), where X = halogen or aryl group. Crystal data: 130 K: 1, Br(Me,N)BB(NMe,)Br, monoclinic, space group PZ,, a = 6.789( 2), b = 7.720(2), c = 10.012(4), p = 103.06(3)', Z = 2, R = 0.048 for 1109 [/ > 2.00(/)] data; 2, Br(Me,N)BB(NMe,)(mes), monoclinic, space group C2/c, a = 24.890(14), b = 7.662(4), c = 18.500(12), p = 116.36(4)', Z = 8, R = 0.063 for 1910 [/ > 2.00(/)] data; 3 (mes)(Me,N)BB(NMe,)(mes), orthorhombic, space group Pbcn, a = 37.983(14), b = 14.1 67(5), c = 12.282(4) A, Z = 12, R = 0.095 for 1829 [/ > 3.0a(/)] data; 4, Ph(Me,N)BB(NMe,)Ph, orthorhombic, space group P2,2,2,, a = 8.579(2), b = 12.920(4), c = 13.925(3) A, Z = 4, R = 0.045 for 1776 [/ > 4.0a(/)] data.Derivatives of diborane(4) (i.e. BzH4) are the simplest catenated boron compounds.',' Yet, in spite of the very large amount of information now available for B-B bonded, three-dimensional polyboron species, diborane(4) species have received relatively little attention. The main reason for this has probably been their perceived instability.'.' For example, diborane(4) itself is stable only when it is complexed with Lewis bases such as amines or ph~sphines.~ In addition, its halide derivatives, e.g. B ~c 1 4 . ~ are difficult to synthesise in large quantities and, with the exception of B2F4,' have marginal stability under ambient conditions. Until recently, the only well known stable diborane(4) compounds were those that involved good n-donor substituents e.g.-NR,, -OR groups (R = alkyl or aryl), a phenomenon that

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