275-8575, Japan § X-ray structure analyses.b S Supporting Information ' INTRODUCTIONThe 16-electron intermediates (R Ru ,R C )-and (S Ru ,R C )-[CpRu-(Prophos)] þ , formed in the dissociation of the RuÀCl bond in solutions of the half-sandwich complexes (R Ru ,R C )-and (S Ru , R C )-[CpRu(Prophos)Cl] at ambient temperatures, retain their pyramidal geometry with the empty site in the remaining coordination position. 1 The degree of retention of configuration at the metal atom in substitution reactions depends on the stability of the unsaturated intermediates toward pyramidal inversion. It is high for a basilica-type energy profile 2 in which the barrier for pyramidal inversion is much larger than the barriers for addition of other ligands, as established for epimerization and ligand exchange in (R Ru ,R C )/(S Ru ,R C )-[CpRu(Prophos)Cl]. 1,2 In the present paper, inferred from the diastereomer ratio of substitution products, we demonstrate that epimerization and ligand exchange in (R Fe , R C )/(S Fe ,R C )-[CpFe(Prophos)NCMe]X (X = I, PF 6 ) follow an energy profile in which the barrier of pyramidal inversion of the intermediates is lower than that of ligand addition to the intermediates (Scheme 1).Synthesis of (R Fe ,R C )/(S Fe ,R C )-[CpFe(Prophos)NCMe]X (X = I, PF 6 ). (R Fe ,R C )/(S Fe ,R C )-[CpFe(Prophos)I] 3 (95:5) and an excess of NH 4 PF 6 were dissolved in acetonitrile and stirred for 1 h at room temperature (Scheme 2). After removal of the solvent the residue was chromatographed at SiO 2 with CH 2 Cl 2 /THF (100:1) or THF. The red-brown band contained (R Fe ,R C )/(S Fe , R C )-[CpFe(Prophos)NCMe]PF 6 (5:95). The preparation of (R Fe ,R C )/(S Fe ,R C )-[CpFe(Prophos)NCMe]I (5:95) was carried out without NH 4 PF 6 . Due to ion exchange effects, it did not elute completely with THF on a SiO 2 column. However, it could be purified by chromatography on Celite. The formulas of Scheme 2 show only (R Fe ,R C )-[CpFe(Prophos)I] and

The chiral-at-metal diastereomers (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] (P−P′ = (R)-Prophos and (R,R)-Norphos, Hal = Cl, Br, and I) were synthesized, separated, and characterized by X-ray crystallography. Halide exchange and epimerization reactions were studied in 9:1 and 1:1 chloroform/methanol mixtures proceeding at room temperature or slightly above according to first-order. The rate-determining step in the Hal exchange reactions was the dissociation of the Ru−Hal bond, forming the pyramidal 16-electron intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+, which maintain the metal configuration. These intermediates can invert their metal configuration or react with nucleophiles with retention of the metal configuration. The measured competition ratios showed that the inversion of the intermediates was slow compared to quenching with nucleophiles, indicating a high pyramidal stability of the 16-electron fragments (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ toward inversion in agreement with a basilica-type energy profile. Stereochemically this implies that substitution reactions in (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal] occur with predominant retention of configuration, however, accompanied by a well-defined share of inversion, a point overlooked when (R Ru,R C)- and (S Ru,R C)-[CpRu(Prophos)Cl] were extensively used as starting materials in the synthesis of a variety of organometallic derivatives. The rates of the approach to the epimerization equilibrium were much smaller than those of the Hal exchange reactions, because in the basilica-type energy profile the intermediates (R Ru,R C)-/(S Ru,R C)-[CpRu(P−P′)]+, formed in the cleavage of the Ru−Cl bond, have to cross another barrier of appreciable height for inversion. Increasing the methanol content of the solvent increased the rates of the Hal exchange and epimerization reactions. Obviously, the pyramidality of the fragments [CpRu(P−P′)]+ is enforced by the small P−Ru−P angles (83° in the Prophos derivatives and 86° in the Norphos derivatives of (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)Hal]). Due to these small angles, intermediates (R Ru,R C)- and (S Ru,R C)-[CpRu(P−P′)]+ resist planarization and thus inversion of the metal configuration.

The compounds [CpFe(Prophos)Cl] and [CpFe(Prophos)I] were prepared in photochemical reactions of [CpFe(CO)2Cl] and [CpFe(CO)2I] with (R)-Prophos. They consist of pairs of R Fe,R C and S Fe,R C diastereomers which only differ in the configuration at the metal atom. The diastereomerically pure compounds (S Fe,R C)-[CpFe(Prophos)Cl] and (R Fe,R C)-[CpFe(Prophos)I], which have the same relative configurations, were isolated. They epimerize via change of the Fe configuration and approach the equilibria (R Fe,R C)-/(S Fe,R C)-[CpFe(Prophos)Cl] = 5/95 and (R Fe,R C)-/(S Fe,R C)-[CpFe(Prophos)I] = 95/5 in first-order reactions with half-lives of 43 min at 20 °C and 50 min at 50 °C in C6D6, respectively. The reaction of (R Fe,R C)-/(S Fe,R C)-[CpFe(Prophos)I] = 95/5 with KCN afforded the cyano complex [CpFe(Prophos)CN] in the diastereomer ratio R Fe,R C/S Fe,R C = 50/50. Both diastereomers (R Fe,R C)- and (S Fe,R C)-[CpFe(Prophos)CN] could be isolated diastereomerically pure. The compounds (R Fe,R C)- and (S Fe,R C)-[CpFe(Prophos)CN] are configurationally stable at the metal center. There is no diastereomer interconversion, not even at higher temperatures. The carbonyl complexes [CpFe(Prophos)CO]I, [CpFe(Prophos)CO]PF6, and [IndFe(Prophos)CO]I were prepared in thermal reactions of [CpFe(CO)2I] and [IndFe(CO2)I] with (R)-Prophos or in an autoclave reaction of [CpFe(Prophos)I]/NH4PF6 with CO under pressure. All the carbonyl complexes are configurationally stable at the metal center. Seven diastereomers were characterized by X-ray crystallography. Including the two diastereomers (R Fe,R C)-[CpRu(Prophos)Br] and (R Fe,R C)-[CpRu(Prophos)I], a conformational analysis of the M-Prophos chelate ring was carried out, resulting in characteristic differences between major and minor diastereomers.