STEREOCHEMICAL LABILITY OF (CO)3FE(MU-ER2)2FE(CO)2PR3 - EVIDENCE FOR 2 DISTINCT CO AVERAGING PROCESSES WITHOUT BRIDGE OPENING

Photochemical incorporation of the phosphines PPhMe2 or PPhBzMe into (CO)3Fe(μ-AsMe2)2Fe(CO)3 forms the title compounds which are shown by 1H and 13C NMR spectra to have stereochemical nonrigidity very similar to that of the parent hexacarbonyl. The arsenide methyl groups average with AG‡298 ≈= 15-16 kcal/mol. The 13C NMR spectrum of (CO)3Fe(μ-AsMe2)2Fe(CO)2(PPhBzMe) exhibits diastereotopic CO groups on the Fe(CO)2L terminus which average with ΔG‡298 = 15.6 kcal/mol. The 13C NMR spectrum of (CO)3Fe(M-PPh(CH2)3-M-PPh)Fe(CO)2(PPhBzMe), in which the μ-phosphide ligands are constrained by a -(CH2)3- bridge, shows no indication of averaging of the diastereotopic CO ligands. These data lead to the conclusions that (1) there is no intramolecular metal-metal CO ligand interchange, (2) there are two processes which average the CO ligands on a given metal terminus, (3) neither of these CO averaging processes is a formai Berry pseudorotation, at least for the Fe(CO)2L terminus, and (4) averaging of the bridge ligand alkyl groups is more likely to arise via a nondissociative molecular deformation than by a bridge-opening process. © 1978, American Chemical Society. All rights reserved.