The rate-determining step in the hydroformylation of 1-octene, catalysed by the rhodium-Xantphos catalyst system, was determined by using a combination of experimentally determined (1)H/(2)H and (12)C/(13)C kinetic isotope effects and a theoretical approach. From the rates of hydroformylation and deuterioformylation, a small (1)H/(2)H isotope effect of 1.2 was determined for the hydride moiety of the rhodium catalyst. (12)C/(13)C isotope effects of 1.012(1) and 1.012(3) for the alpha-carbon and beta-carbon atoms of 1-octene were determined, respectively. Both quantum mechanics/molecular mechanics (QM/MM) and full quantum mechanics calculations were carried out on the key catalytic steps, for "real-world" ligand systems, to clarify whether alkene coordination or hydride migration is the rate-determining step. Our calculations (21.4 kcal mol(-1)) quantitatively reproduce the experimental energy barrier for CO dissociation (20.1 kcal mol(-1)) starting at the (bisphosphane)RhH(CO)(2) resting state. The barrier for hydride migration lies 3.8 kcal mol(-1) higher than the barrier for CO dissociation (experimentally determined trend approximately 3 kcal mol(-1)). The computed (1)H/(2)H and (12)C/(13)C kinetic isotope effects corroborate the results of the energy analysis.

Reaction of the transient phosphinidene complexes R‐PW(CO)5 with N‐substituted‐diphenylketenimines leads unexpectedly to the novel 2‐aminophosphindoles, as confirmed by an X‐ray crystal structure determined for one of the derivatives. Experimental evidence for a methylene‐azaphosphirane intermediate was found by using the iron‐complexed phosphinidene iPr2N‐PFe(CO)4, which affords the 2‐aminophosphindole together with the novel methylene‐2,3‐dihydro‐1H‐benzo[1,3]azaphosphole. Analysis of the reaction pathways with DFT indicates that the initially formed methylene‐azaphosphirane yields both phosphorus heterocycles by way of a [1,5]‐ or [1,3]‐sigmatropic shift, respectively, followed by a H‐shift. Strain underlies both rearrangements, which causes these remarkably selective conversions that can be tuned by changing the substituents.