The complexes [RhBr(COD)]2(μ-di-NHC), in which di-NHC represents the di-N-heterocyclic carbenes MeCCmeth (1,1′-methylene-3,3′-dimethyldiimidazol-2,2′-diylidene), tBuCCmeth (1,1′-methylene-3,3′-di-tert-butyldiimidazol-2,2′-diylidene), and MeCCeth (1,2-ethylene-3,3′-dimethyldiimidazol-2,2′-diylidene), have been prepared from the reactions of the corresponding diimidazolium bromide salts with the acetate-bridged complex [Rh(μ-OAc)(COD)]2. The analogous complex containing the tBuCCeth group (1,2-ethylene-3,3′-di-tert-butyldiimidazol-2,2′-diylidene) could not be prepared by this route, but the related chloro complex [RhCl(COD)]2(μ- tBuCCeth) was prepared by prior double deprotonation of the corresponding diimidazolium salt and reaction of the resulting dicarbene with [RhCl(COD)]2. Reaction of the above MeCCeth and tBuCCeth complexes with CO yields the corresponding dicarbene-bridged carbonyl products [RhX(CO)2]2(μ-di-NHC) (X = Cl, Br), while reaction of the MeCCmeth and tBuCCmeth complexes with CO instead yielded the products [Rh(CO)2(η1:η1-di-NHC)]+, containing chelating dicarbenes. The X-ray structure determinations of several of the above dicarbene-bridged species have been carried out and show metal−metal separations of greater than 6.6 Å in all cases. Reaction of all of the above COD complexes with dppm (Ph2PCH2PPh2) followed by CO yields the dicarbene- and dppm-bridged A-frame species [Rh2(CO)2(μ-X)(μ-di-NHC)(dppm)]+ (X = Cl, Br), in which the larger bite of the dicarbenes, compared to that of dppm, induces strain within the complexes. Introduction of the bridging dppm and bromide groups lowers the Rh−Rh separation to about 3.3 Å. Replacement of the bridging bromide by hydroxide in the MeCCeth complex yields [Rh2(CO)2(μ-OH)(μ-MeCCeth)(dppm)]+, in which the bridging hydroxide can be used to deprotonate a monoimidazolium iodide to give [Rh2Br(CO)2(IMe)(μ-MeCCeth)(dppm)][I] (IMe = 1,3-dimethylimidazol-2-ylidene). In this species the bromide is on one metal while the monocarbene occupies the other, opening up the dicarbene bite to 5.18 Å and the metal−metal separation to 4.07 Å. Attempts to better match the larger bite of the dicarbenes by substituting dppm with dppe (Ph2PCH2CH2PPh2) did not give a dppe-bridged product and, instead, yielded [(Rh(CO)(dppe))2(μ-MeCCeth)][CF3SO3]2 in which the dppe groups are chelating.

Reactions of the bis-ethylene complex [RhOs(η2-C2H4)2(μ-CO)2(dppm)2][CF3SO3], with CO, MeCN, PMe3, IMe4, and [PPN][Cl] (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; PPN+ = bis(triphenylphosphine)iminium) yield the monoethylene products [RhOsL(C2H4)(μ-CO)2(dppm)2][CF3SO3] (L = CO (2), MeCN (3), PMe3 (4), IMe4 (5)) and [RhOsCl(C2H4)(μ-CO)2(dppm)2] (6), respectively, by substitution of the Rh-bound ethylene ligand. In the case of L = CO, carbon monoxide was added at −78 °C, since at higher temperatures substitution of both ethylene ligands occurred to give the known product [RhOs(CO)4(dppm)2][CF3SO3]. For L = IMe4, the stoichiometry had to be carefully controlled, since in the presence of more than 1 equiv of IMe4 deprotonation of a dppm methylene group by IMe4 occurred to yield [RhOs(IMe4)(C2H4)(μ-CO)2(Ph2PCHPPh2)(dppm)]. The IMe4 product (5) could also be obtained in THF solution by substitution of the acetonitrile ligand in compound 3. However, when this reaction was carried out in acetonitrile, deprotonation of the acetonitrile ligand by IMe4 occurred to give the acetonitrilide intermediate [RhOs(NCCH2)(C2H4)(μ-CO)2(dppm)2], which rapidly coupled with another 1 equiv of 3 to yield the enamino−nitrile-bridged product [(RhOs(C2H4)(μ-CO)2(dppm)2)2(μ-NHC(CH3)CHCN)][CF3SO3].

The reaction of [RhOs(CO)(3)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] (dppm = μ-Ph(2)PCH(2)PPh(2)) with 1,3,4,5-tetramethylimidazol-2-ylidene (IMe(4)) results in competing substitution of the Rh-bound carbonyl by IMe(4) and dppm deprotonation by IMe(4) to give the two products [RhOs(IMe(4))(CO)(2)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] and [RhOs(CO)(3)(μ-CH(2))(μ-κ(1):η(2)-dppm-H)(dppm)] [3; dppm-H = bis(diphenylphosphino)methanide], respectively. In the latter product, the dppm-H group is P-bound to Os while bound to Rh by the other PPh(2) group and the adjacent methanide C. The reaction of the tetracarbonyl species [RhOs(CO)(4)(μ-CH(2))(dppm)(2)][CF(3)SO(3)] with IMe(4) results in the exclusive deprotonation of a dppm ligand to give [RhOs(CO)(4)(μ-CH(2))(μ-κ(1):κ(1)-dppm-H)(dppm)] (4) in which dppm-H is P-bound to both metals. Both deprotonated products are cleanly prepared by the reaction of their respective precursors with potassium bis(trimethylsilyl)amide. Reversible conversion of the μ-κ(1):η(2)-dppm-H complex to the μ-κ(1):κ(1)-dppm-H complex is achieved by the addition or removal of CO, respectively. In the absence of CO, compound 3 slowly converts in solution to [RhOs(CO)(3)(μ-κ(1):κ(1):κ(1)-Ph(2)PCHPPh(2)CH(2))(dppm)] (5) as a result of dissociation of the Rh-bound PPh(2) moiety of the dppm-H group and its attack at the bridging CH(2) group. Compound 4 is also unstable, yielding the ketenyl- and ketenylidene/hydride tautomers [RhOs(CO)(3)(μ-κ(1):η(2)-CHCO)(dppm)(2)] (6a) and [RhOs(H)(CO)(3)(μ-κ(1):κ(1)-CCO)(dppm)(2)] (6b), initiated by proton transfer from μ-CH(2) to dppm-H. Slow conversion of these tautomers to a pair of isomers of [RhOs(H)(CO)(3)(μ-κ(1):κ(1):κ(1)-Ph(2)PCH(COCH)PPh(2))(dppm)] (7a and 7b) subsequently occurs in which proton transfer from a dppm group to the ketenylidene fragment gives rise to coupling of the resulting dppm-H methanide C and the ketenyl unit. Attempts to couple the ketenyl- or ketenylidene-bridged fragments in 6a/6b with dimethyl acetylenedicarboxylate (DMAD) yield [RhOs(κ(1)-CHCO)(CO)(3)(μ-DMAD)(dppm)(2)], in which the ketenyl group is terminally bound to Os.

The complexes [RhOsL(CO)3(dppm)2][CF3SO3] (L = IMe4 (1,3,4,5-tetramethylimidazol-2-ylidene) (6), PMe3 (8); dppm = μ-Ph2PCH2PPh2) were prepared by substitution of a carbonyl ligand in [RhOs(CO)4(dppm)2][CF3SO3]. Reaction of 6 with additional IMe4 resulted in deprotonation of a dppm ligand, yielding [RhOs(IMe4)(CO)3(μ-κ1:κ1-Ph2PCHPPh2)(dppm)] (7). Although reaction of 8 with diazomethane at −78 °C yielded the known methylene-bridged [RhOs(PMe3)(CO)3(μ-CH2)(dppm)2][CF3SO3] (3), compound 6 was unreactive toward diazomethane over a wide temperature range. The methylene-bridged species [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] (9) was obtained by reaction of [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3] with IMe4, although [RhOs(CO)3(μ-CH2)(μ-κ1:η2-Ph2PCHPPh2)(dppm)] (10) was also obtained by competing dppm deprotonation by IMe4. Protonation of [RhOsL(CO)2(μ-CH2)(dppm)2][CF3SO3] (L = IMe4 (9), PMe3 (11)) with triflic acid at −78 °C yielded two isomers in each case. The more abundant isomer, [RhOsL(CO)2(μ-CH3)(dppm)2][CF3SO3]2, has a bridging agostic methyl group, while the minor isomer has a terminal, Os-bound methyl group. Upon warming, both isomers transformed to species having an Os-bound methyl group and a coordinated triflate ion, subsequently rearranging to the thermodynamic products [RhOsL(CO)2(μ-H)(μ-CH2)(dppm)2][CF3SO3]2 near ambient temperature. Attempts to prepare an IMe4-containing methyl species directly via triflate ion substitution in [RhOs(CH3)(OSO2CF3)(CO)3(dppm)2][CF3SO3] by IMe4 instead resulted in deprotonation of the methyl group to give the known product [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3]. Addition of methyl triflate to 6 gave no reaction, but protonation of 6 with triflic acid at −78 °C yielded the kinetic isomer of [RhOsH(IMe4)(CO)3(dppm)2][CF3SO3]2, in which the hydride is terminally bound to Os, and warming this product to ambient temperature resulted in rearrangement to the hydride-bridged, thermodynamic isomer.