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Rh-S-H are -14.5°in 2). Thus, the hydrogens linked to one metal and to one sulfur remain in close proximity.A CH2C12 solution of 2 under a stream of argon loses 2 mol of H2 (as determined by GC) and transforms into [(triphos)Rh-(M-S)2Rh(triphos)](BPh4)2 (3) (31P|'H| NMR (CD3COCD3, 293 K) 24.02 ppm (/(P-Rh) = 102 Hz, doublet, triphos)). Brown crystals are precipitated by addition of ethanol. A stream of H2 turns a solution of 3 to the original pink color, and the complex 2 is obtained quantitatively in 4 h. A preliminary X-ray analysis on 3 has confirmed the structural formulation given in Scheme I.The ability of 3 to activate homodinuclear molecules shows up in the reaction with oxygen. When 02 is bubbled throughout a CH2C12 solution of 3, red crystals of the µ-SO complex [(triphos)Rh(/u-SO)2Rh(triphos)](BPh4)2 (4) are obtained quantitatively. The crystal structure and characterization of 4, prepared by a different synthetic strategy, have recently been reported by (3) Ghilardi, C.

The reaction of thermally generated cobalt atoms with hexaborane(lO), bis(trimethyhilyl)acetylene, and cyclopentadiene has been found to give as major products the cobaltacarborane clusters 5:1',2'-[ 147-C5H5)Co-2,3-(Me3Si)zCzB4H3][B2H5], I, 1-(q-C5H5)Co-4,6-(Me3Si)2c2B6H6, 11, and l-(tpC5H5)Co-4,5-(Me3Si)2C2B6H6, 111, along with smaller amounts of 5-(q-C5H5)C~-l,8-(Me3Si)2C2B5H5, IV, and 1-(q-C5H5)Co-2,3-(Me3Si)2C2B4H4, V. The spectroscopic data for compounds 11,111, IV, and V are consistent with their formulations as closo polyhedral cage systems in which one bis(trimethylsily1)acetylenic unit has been inserted into the cage framework. A single-crystal X-ray study of I has revealed, however, that it has a unique bridged structure composed of a ~loso-l-(q-C~H~)Co-2,3-(Me~Si)~C~B~H~-sandwich complex which has an exopolyhedral -B2H5 group bound to the 5-boron cage atom by means of a boron-boron-boron three-center bond. Compound I can thus be considered a bridge-substituted organometallic derivative of diborane. Crystal data for I: s ace group C2/c; 2 = 8; a = 29.890 (9) A, b = 9.213 (2) A, c = 15.127 (6) A, / 3 = 91.57 (3)O; V = 4164.4 13. The structure was refined by full-matrix least-squares techniques to a final R of 0.033 and R, of 0.042 for the 4005 unique reflections which had F,2 > 30(F$). IntroductionWe have previously reported that the reaction of metal atoms with boron hydrides, acetylenes, and either cyclopentadiene or arenes can lead to the incorporation of a n acetylenic unit into the metallaboron cluster framework to yield (+25H5)metallacarborane2 or ($-C6R6)metallacarborane c~m p l e x e s .~ In general, the yields of metalcontaining products in these reactions were found to be low owing, in part, to competing metal atom promoted alkyne oligomerizations. For example, in reactions employing 2 -b~t y n e ,~*~ hexamethylbenzene was found to be the major product. This observation prompted our investigation of the reactions of metal atoms and boron hydrides with alkynes, such as bis(trimethylsilyl)acetylene, which would be more resistant to cyclization, with the aim

The reaction of thermally generated iron atoms with pentaborane(9), toluene, and 2-butyne was found to yield as the major product the (x-arene)ferracarborane sandwich complex l-[V-C6(CH3)6]Fe-2,3-(CH3)2C2B4H4, I, along with smaller amounts of the four-carbon metallacarborane complexes 1-[t?6-C6-(CH3)6]Fe-4,5,7,8-(CH3)4C4B3H3, II, l-[^6-CH3C6H6]Fe-4,5,7,8-(CH3)4C4B3H3, III, and 2-["6-CH3C6H5]Fe-6,7,9,10-(CH3)4C4B5H6, IV. Reactions carried out in the absence of toluene also yielded I and II but in reduced yields. The structures of II and IV were determined by single-crystal determinations. Compound II was shown to have an arachno-type cage geometry derived from a bicapped-square antiprism missing two vertices, with the iron atom occupying the five-coordinate 1-position in the cage and the four carbons occupying positions on the open face of the cage. An unusually short carbon-carbon bond distance of 1.367 (4) A was observed for the non-metal-bonded carbons in the cage and is thought to result from localized multiple bond character between these carbons. Crystal data for II: space group 2 / , Z = 4, a = 8.753(3) Á, b = 10.190 (2) Á, c = 22.118 (3) Á, ß = 95.08 (2)°, V = 1965 Á3 4. The structure was refined by full-matrix least squares to a final R of 0.060 and R" = 0.066 for the 2597 unique reflections which had F02 > 3o(F2). Compound IV was shown to be an analogue of decaborane( 14) in which the iron atom occupies the five-coordinate 2-position in the cage and the carbon atoms occupy the 6,7-and 9,10-positions on the open face. Crystal data for IV: space group Pi, Z = 2, a = 9.368 (5) Á, b = 11.399 (4) Á, c = 8.708 (4) A, a = 90.84 (4)°, ß = 116.51 (5)°, = 94.03 (4)°, V = 829 Á3. The structure was refined by full-matrix least squares to a final R of 0.074 and Pw = 0.065 for the 760 unique reflections which had F2 > 3a{F2). The iron to arene bonding in both II and IV appears normal and is consistent with a symmetrical ^-coordination. Reactions of iron atoms with hexaborane(lO), toluene, and 2-butyne were also found to give low yields of compounds I, II, and III.

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