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 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.

The compounds (R Fe ,R C )-/(S Fe ,R C )-[CpFe(Prophos)NCMe]X (X = I, PF 6 ), configurationally labile at the metal center, were used in the MeCN/ligand exchange reactions with cyclohexyl isocyanide (CyNC) and tert-butyl isocyanide ( t BuNC). Kinetic measurements showed that the MeCN/CyNC exchange in diastereomerically pure (S Fe ,R C )-[CpFe(Prophos)-NCMe]X proceeded via the slow S N 1-type dissociation of the Fe−NCMe bond, already observed in the MeCN/phosphite exchange reactions. The product (R Fe ,R C )-/(S Fe ,R C )-[CpFe(Prophos)CNCy]X (X = I, PF 6 ) was formed in diastereomer ratios between 40:60 and 60:40. However, specific for the MeCN/CyNC exchange in (S Fe ,R C )-[CpFe(Prophos)NCMe]PF 6 , in some of the samples a fast initial reaction interfered, initiated by traces of oxygen, which oxidized the cation in (S Fe ,R C )-[CpFe(Prophos)NCMe]PF 6 to (S Fe ,R C )-[CpFe(Prophos)NCMe] 2+ . This dipositive cation started an electrocatalytic chain reaction, producing (R Fe ,R C )-/(S Fe ,R C )-[CpFe(Prophos)CNCy]PF 6 with a high stereoselectivity of 2:98 in favor of (S Fe ,R C )-[CpFe(Prophos)CNCy]PF 6 . Deactivation processes terminated the chain reaction, depending on the varying amounts of (S Fe ,R C )-[CpFe(Prophos)NCMe] 2+ present in the system. Larger amounts of oxygen or oxidants, such as I 2 and AgPF 6 , caused immediate complete conversion to (R Fe ,R C )/(S Fe ,R C )-[CpFe(Prophos)CNR]PF 6 in a diastereomer ratio of 2:98. In contrast to the hexafluorophosphate salt, addition of a crystal of iodine did not initiate the chain reaction in the iodide salt [CpFe(Prophos)NCMe]I, because I 2 added to I − to form I 3 − , which did not oxidize the cation of [CpFe(Prophos)NCMe]I.Instead, there was slow conversion according to the dissociative pathway. The correlation between the configuration of (R Fe ,R C )and (S Fe ,R C )-[CpFe(Prophos)CNCy]X and the conformation of the Fe-Prophos chelate ring on the one hand and the correlation with the P−P coupling constants of the Prophos ligand on the other hand was corroborated.