Reaction of the methylene-bridged complex [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3] (dppm = μ-Ph2PCH2PPh2) with dimethyl acetylenedicarboxylate or hexafluorobutyne results in alkyne insertion into the Rh−CH2 bond, yielding [RhOs(CO)3(μ-η1:η1-RCC(R)CH2)(dppm)2][CF3SO3] (R = CO2Me (5), CF3 (6)). Carbonyl removal from each of these species results in a Rh-bound carbonyl being replaced by a triflate ion. Reaction of 5 with diazomethane results in CH2 insertion into the Rh−C(R) bond with accompanying C−H activation of the original osmium-bound CH2 group, yielding [RhOs(CO)3(μ-η1:η1-CH2C(R)C(R)CH)(μ-H)(dppm)2][CF3SO3] (R = CO2Me). Reaction of the alkyne-bridged species [RhOs(CO)3(μ-η1:η1-RCCR)(dppm)2][CF3SO3] (R = CO2Me) with diazomethane at −78 °C yields [RhOs(CO)3(μ-η1:η1-CH2C(R)CR)(dppm)2][CF3SO3] (12), which upon warming to ambient temperature, in the absence of diazomethane, undergoes alkyne deinsertion to yield [RhOs(CO)3(η2-RC⋮CR)(μ-CH2)(dppm)2][CF3SO3] (13). This species and the inserted product (12) appear to be in equilibrium, so reaction of 13 with diazomethane proceeds via 12 to give [RhOs(CO)3(η1:η1-CH2CH2C(R)CR)(dppm)2][CF3SO2] (14), in which the hydrocarbyl fragment is chelating to Os and interacting with Rh via an agostic interaction with one of the Os-bound CH2 hydrogens. Upon standing for weeks, or more rapidly upon refluxing, compound 14 transforms into an isomer (15) in which the C4 unit has migrated from sites on Os adjacent to Rh to sites on Os remote from Rh. An X-ray structure determination of compound 15 as the BF4 - salt confirms the geometry of this final species. Reaction of the hexafluorobutyne-bridged species [RhOs(CO)3(μ-η1:η1-RCCR)(dppm)2][CF3SO3] (R = CF3) with diazomethane does not give a product analogous to that described for dimethyl acetylenedicarboxylate, but yields the isomer 6. These carbon−carbon bond forming processes are discussed.

The reactions of allene, methylallene, and 1,1-dimethylallene with [RhM(CO) 4 (µ-CH 2 )-(dppm) 2 ][X] (M ) Os, X ) BF 4 (1); M ) Ru, X ) CF 3 SO 3 (3); dppm ) µ-Ph 2 PCH 2 PPh 2 ) in the presence of 1 equiv of trimethylamine oxide yield a variety of products. Under these conditions compounds 1 and 3 react with allene to yield the trimethylenemethane-bridged products [RhM(CO) 2 (µ-η 3 :η 1 -C(CH 2 ) 3 )(dppm) 2 ][X] (M ) Os ( 5), Ru (6)), in which the trimethylenemethane fragment is η 1 -bound to the group 8 metal by one methylene group and η 3 -bound to Rh by the rest of the fragment. These compounds react with CO and PMe 3 to yield, in which L is coordinated to Rh and the η 3 :η 1 -binding mode of trimethylenemethane is retained. Although attack of L occurs at Rh for the Rh/Ru species, carbonyl attack in the Rh/Os compound initially occurs at Os with subsequent scrambling throughout the three carbonyl sites; attack of PMe 3 on the Rh/ Os compound occurs at Rh. Methylallene reacts with 1 and Me 3 NO to give a species analogous to 5 and 6, having the methyl substituent on one of the terminal carbons of the allylic functionality. This methyl group is involved in an agostic interaction with Rh, which can be displaced by CO. The tricarbonyl complex [RhOs(CO) , in which the dimethylvinyl group is adjacent to Rh, exo to the RhCCH 2 CH 2 Os dimetallacycle. At ambient temperature this species decomposes to give 1,1-dimethyl-1,3butadiene. Compound 3 reacts with 1,1-dimethylallene in the presence of Me 3 NO to yield [RhRu(CO) 2 ((CH 3 ) 2 CCCH 2 CH 2 CO)(dppm) 2 ][CF 3 SO 3 ] (14), in which [2+1+1] coupling of the allene, the bridging methylene group, and a coordinated carbonyl has occurred. The dative bond from the bridging acyl moiety of 14 to Ru is displaced in reactions with H 2 and CO to give [RhRu(CO) 2 (C(O)CH 2 CH 2 C(dC(CH 3 ) 2 ))(µ-H) 2 (dppm) 2 ][CF 3 SO 3 ] and [RhRu(CO) 3 (C(O)CH 2 -CH 2 C(dC(CH 3 ) 2 ))(dppm) 2 ][CF 3 SO 3 ], respectively. A rationalization of this chemistry is presented.