Recently, Brown and co-workers reported the use of the bromonium ion of adamantylideneadamantane, as the triflate salt ([AdAdBr](OTf)), to transfer Br + to acceptor olefins. 1 On the other hand, work from this laboratory has shown that reaction of iodine with Os 3 (CO) 12 proceeds via the [Os 3 (CO) 12 -(I)] + cation, and it was suggested that bromination of Os 3 (CO) 12 proceeded through a similar intermediate. 2 Here we report the use of the bromonium ion reagent to prepare the [M 3 (CO) 12 -(Br)] + (M ) Ru, Os) ions and the determination of their crystal structures. These ions are the first halonium ions of metal carbonyl cluster compounds to be structurally characterized. The two cations have markedly different structures: the ruthenium derivative can be considered as an organometallic analogue of the bromonium ion of ethylene, [C 2 H 4 Br] + , whereas the osmium cation can be thought of as the corresponding ion of cyclopropane, [C 3 H 6 Br] + . Furthermore, evidence is presented that the form of [Os 3 (CO) 12 (Br)] + characterized here is not an intermediate in the bromination of Os 3 (CO) 12 .Addition of [AdAdBr]{B[C 6 H 3 (CF 3 ) 2 ] 4 } 3 to M 3 (CO) 12 (M ) Ru, Os) in CH 2 Cl 2 at or below room temperature readily afforded [M 3 (CO) 12 (Br)]{B[C 6 H 3 (CF 3 ) 2 ] 4 }, which on addition of hexane formed air-stable, red-orange (M ) Ru) or pale yellow (M ) Os) crystals. 5 The structure of the ruthenium cation (1) as found in the X-ray structure determination of [Ru 3 (CO) 12 -(Br)]{B[C 6 H 3 (CF 3 ) 2 -3,5] 4 } 6 is shown in Figure 1. It consists of a bent chain of three ruthenium atoms (Ru(1)Ru(2)Ru(3) ) 157.19(4)°), one Ru-Ru bond of which is bridged by the