A series of novel half-sandwich M(I) and M(III) complexes (M = Co, Rh) bearing the N-heterocyclic carbene ligand 1,3-dimesitylimidazol-2-ylidene (IMes) have been prepared and characterized. Thus, (eta5-C(5)R(5))M(IMes)(C(2)H(4))(M = Co, Rh; R = H, Me) were obtained from the corresponding bis(ethene) complexes (eta5-C(5)R(5))M(C(2)H(4))(2), except for CpRh(IMes)(C(2)H(4)) which was prepared via the novel 16-electron Rh(I) compound Rh(IMes)(C(2)H(4))(2)Cl. The carbonyl compounds (eta5-C(5)R(5))Co(IMes)(CO)(R = H, Me) were synthesized by thermal CO substitution of (eta5-C(5)R(5))Co(CO)(2). A diamagnetic, apparently 16-electron Co(III) compound [CpCo(IMes)I](+)[I(3)(-)] was obtained from CpCo(IMes)(CO) and I(2). Finally, Co(III) and Rh(III) complexes CpCo(IMes)Me(2) and Cp*Rh(IMes)Me(2) were prepared by methylation of [CpCo(IMes)I](+)[I(3)(-)], and ligand exchange at Cp*Rh(Me(2)SO)Me(2), respectively. The molecular structures of CpCo(IMes)(CO), CpRh(IMes)(C(2)H(4)), Cp*Rh(IMes)(C(2)H(4)), and Cp*Rh(IMes)Me(2) were determined by single crystal X-ray diffraction. Steric and electronic factors imposed by the strongly donating and sterically demanding IMes ligand are discussed on the basis of X-ray crystallographic, NMR, and IR spectroscopic analyses. Very poor correlations are found between values for (1)J(Rh-C(carbene)) and dRh-C(carbene) data for Rh(i) N,N-heterocyclic carbene complexes including literature data and this work.

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The trimethylrhodium(III) complex CnRhMe(3) (1, Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane) undergoes a nearly reversible one-electron oxidation at E degrees = -0.15 V vs Cp(2)Fe/Cp(2)Fe(+) (cyclic voltammetry, 1.0 V/s) in acetonitrile/0.1 M Bu(4)NPF(6). Preparative electrolysis as well as homogeneous oxidations with substituted ferricinium salts gives a mixture of CnRhMe(2)(NCMe)(+) (2) and CnRhMe(NCMe)(2)(2+) (3), the 2:3 ratio being independent of the nature of the oxidant. In addition, the reactions yielded ethane, mostly by intramolecular elimination. An investigation of the kinetics of the reaction of 1(*)(+) by derivative cyclic voltammetry revealed a unimolecular reaction (DeltaH() = 57.0 +/- 0.9 kJ/mol, DeltaS() = -35.4 +/- 3.0 J/(K.mol), k(20 degrees C) = 5.9 s(-)(1)) with negligible solvent effects (MeCN vs CH(2)Cl(2)). It is proposed that 1(*)(+) eliminates ethane to generate the formally 15-electron CnRhMe(*)(+) in the rate-determining step. The final Rh-containing products are likely formed from this species and 1(*)(+).

When Cp*Rh(C(2)H(4))(2)H(+) (2) is exposed to C(2)H(4) in the gas phase, inside the cell of an FT-ICR mass spectrometer, the most notable feature is the lack of any bimolecular reactivity. Collisional activation of 2 leads to ethylene loss and formation of Cp*Rh(C(2)H(4)-mu-H)(+) (3). In contrast to the reactivity of 2 in solution, ethylene dimerisation is negligible in the gas phase. Coordinatively unsaturated 3, rather than 2, is the major species in which reactivity is observed to occur. Compound 3 reacts with ethylene in three parallel processes: (a) Slow addition of ethylene to give 2; (b) rapid, intermolecular hydrogen atom exchange (monitored in separate reactions with free C(2)D(4) to give 3-d(1-5)); (c) ligand substitution of ethylene in 3. DFT calculations reproduce these observations, showing low barriers for hydrogen scrambling, high barrier to ligand loss in 2, and even higher barriers to elimination of either H(2) or ethane. Mechanistic models for the elimination and scrambling processes are discussed.