Breath‐taking activation: Stereoregular carbene polymerization proceeds via cationic [(allyl)RhIII–polymeryl]+ species. These are most efficiently generated by oxygenation of the [(diene)RhI] precatalysts, which involves an unusual rearrangement of 2‐rhodaoxetane intermediates. This discovery gives detailed insight in the reaction mechanism.

Treatment of bis(2-picolyl)amine (bpa) with [{Rh(nbd)(mu-OMe)}2] leads to the unexpected and unique redox asymmetric dinuclear Rh-I,Rh+I complex [{Rh(nbd)}2(bpa-2H)] (2) with a pi-coordinating imine bound to a tetrahedral low valent rhodate(-I). Mono-oxygenation of the deprotonated bpa ligand in 2 by O2 leads to the mononuclear carboxamido complex [Rh(nbd)(bpam-H)] (3) (bpam = N-(2-picolyl)picolinamide). The second O atom of O2 ends up as a hydroxo fragment in [{Rh(nbd)(mu-OH)}2].

Replacement of the olefin in the complexes [{Rh(μ-RPz)(C2H4)2}2] (RPz = pyrazolate (Pz) (1), 3-methylpyrazolate (MePz) (2), 3,5-dimethylpyrazolate (Me2Pz) (3)) by tert-butyl isocyanide gives [{Rh(μ-RPz)(CNBut)2}2] (4, 5, 6) respectively. Complex 4 can alternatively be prepared from [{Rh(μ-Pz)(cod)}2] (cod = 1,5-cyclooctadiene) (7) leading to a chemical equilibrium between 4, 7, and the intermediate [(cod)Rh(μ-Pz)2Rh(CNBut)2] (8), which has been also isolated. Kinetic studies on this apparent ligand redistribution reaction, leading to 8, show that it follows a second order rate, giving the activation parameters ΔH ≠ = 21.8 kcal·mol-1, ΔS≠ = −7.4 eu, and ΔG ≠298 = 24.0 kcal·mol-1, suggesting that the dinuclear complexes are the active species, and no fragmentation seems to occur. The molecular structures of complexes 3, 4, 8, and [{Rh(μ-Pz)(CO)2}2] (9) determined by X-ray diffraction show discrete dinuclear complexes with the six-membered “Rh(μ-Pz)2Rh” ring showing a boatlike conformation. Complexes 3 and 4 present the shortest and the largest intermetallic nonbonding separations, 3.0961(2) and 3.8995(6) Å, respectively, so far reported in di-μ-pyrazolato-dirhodium(I) complexes. In addition, complex 3 shows the shortest olefinic CC distance found in ethylene rhodium complexes and accordingly a very low activation energy, 10.0 kcal·mol-1, for the rotation of the ethylene ligands. Complex 8 undergoes two independent intramolecular fluxional processes associated to the ring inversion of the six-membered “Rh(μ-Pz)2Rh” metallacycle and to a σ-1,2-metallotropic shift showing activation parameters ΔH ≠ = 15.0 kcal·mol-1, ΔS ≠ = −1.7 eu and ΔH ≠ = 19.7 kcal·mol-1, ΔS ≠ = 4.8 eu, respectively. The first movement is influenced by medium effects and may be restricted by the ancillary ligands and the substituents on the pyrazolate rings in other bis(μ-pyrazolato) complexes. Finally, the intermetallic distance in dinuclear pyrazolato complexes is analyzed in terms of steric and electronic factors.