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compatible with a migration of methyl to give an intermediate analogous to ( I ) , but only with a CO-insertion to give the q2-acetyl intermediate (4). The formation of (4) from (5) can thus be envisioned in terms of the CO ligand being inserted into the Fe-CH3 bond, and the oxygen atom swinging into the position previously occupied by the carbonyl Catom. In the presence of CO, replacement of the acetyl oxygen in the intermediate (4) by the newly entering CO occurs with stereospecific formation of (6). The intermediate (4) is formed from (5) even at room temperature and is configurationally stable under the conditions of the carbonylation.This result is remarkable, because in the photochemical decarbonylation of (8) in tetrahydrofuran, the species (7') with the opposite configuration to (7) at the Fe-atom, is formed, consistent with the concept of a methyl migration into the position vacated by photochemical cleavage of the CO ligand[7b1. It follows, therefore, that the photochemical reaction cannot involve an T*-acetyl intermediate, whose carbonylation according to the principle of microscopic reversibility should give the product (7)-and not (T), as observed-in view of the stereochemistry of the reaction (5) + (6).(5) epimerizes to (5' ) at elevated temperatures. In benzene at 70 "C, approach to the diastereomeric equilibrium (5)&(5') (65:35) is a first order reaction (half-life 81 min) which can be followed NMR spectroscopicallyI'.61. Within the limit of error an addition of the aminophosphane contained in (5) and (5') does not change the rate of the reaction. The epimerization (5) P (5') could be ascribed to a pseudorotation-like rearrangement of the q2-acetyl ligand in (4) and (4'), e.g. by rotation about 180".Reaction of (5) with the isocyanide CNR*= ( -)-CN-(s)-CH(CH3)(C6H5) instead of CO in benzene requires about six hours' heating at 70 "C. The product C5HsFe(COCH3)(CNR*)L has an optical purity of only 50%, since the change of configuration in the intermediate (4) a (4) competes with the trapping reaction (4) + CNR*.The carbonylation of (5), on the other hand, leads stereospecifically to (6) at room temperature, before the intermediate (4) can change its configuration. The variable stereoselectivities observed by Floods1 in the carbonylation of (7) to (8) might be due to prior change of configuration (7)$(7') at the Fe-atom or, in particular in hexamethylphosphoric triamide, to SN2-inversion at the Fe-atomLs1.Summarizing: Although the fragments (C0)3Mn and C5H5Fe, and thus also the compounds (C0)5MnCH3 and C5HSFe(C0)2CH3 are isoelectronic, their carbonylation proceeds via different routes and via different intermediates. This also applies in the case of the photochemical and thermal variants of the carbonylation of CSHsFe(CO)(CH3)L and of the decarbonylation of C5H,Fe(CO)(COCH3)L.

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Hexachlorcyclohexaphosphan (1) und Hexabromcyclohexaphosphan (2) werden bei der partiellen Enthalogenierung von Phosphor(III)-chlorid bzw. Phosphor(I1I)-bromid mit Magnesium oder Lithiumhydrid in polaren Solventien gebildet. Sie sind als erste perhalogenierte Cyclophosphane nur in verdunnter L6sung bei tiefer Temperatur bestandig und konnten massenspektrometrisch und 3'P-NMR-spektroskopisch charakterisiert werden. Wie das schon bekannte Hexaphenylcyclohexaphosphan, aber im Unterschied zur ,,Stammverbindung" PaHc enthalten 1 und 2 ein Phosphor-Sechsringgeriist, das offenbar durch elektronegative Substituenten stabilisiert wird.

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