The reaction of 1,2-{Cp*RuH} 2 B 3 H 7 (1), Cp* ) η 5 -C 5 Me 5 , with Co 2 (CO) 8 in hexane yields two major ruthenaborane products, nido-1,2-{Cp*RuH} 2 -3-Co(CO) 4 B 3 H 4 (2) and arachno-{Cp*RuH}(CO)B 3 H 7 (3), as well as two other ruthenaboranes in low yield. In toluene, benzene, and THF one of these minor ruthenaborane products is found with an enhanced yield intermediate between those of 2 and 3. This compound has been isolated and characterized in solution and the solid state as the 8 skeletal electron pair (sep) arachno 1,2-{Cp*Ru} 2 (CO) 2 B 3 H 7 (4) and shown to convert into a second isomeric form of similar stability. One isomer constitutes a diruthenium analogue of 8-sep pentaborane(11), whereas the other mimics the geometry of a known metal cluster with 8 sep. The variation in product concentrations in benzene, including [Cp*Ru(CO) 2 ] 2 , {Cp*Ru(CO) 2 }Co(CO) 4 , and HCo(CO) 4 , as a function of time and Co 2 (CO) 8 /1 ratio has been determined by 1 H and 11 B NMR spectroscopy and interpreted in terms of a reaction pathway featuring competitive Co(CO) 4 radical addition to ruthenium to yield 2 plus 3 and to boron to yield 4. The apparent solvent dependence of product distribution is attributed to impurity-initiated decomposition of the boron-based radical in hexane.
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