LIGAND-SUBSTITUTION PROCESSES IN TETRANUCLEAR CARBONYL CLUSTERS .5. A KINETIC AND INFRARED INVESTIGATION OF 13CO INCORPORATION INTO CO4(CO)9(MU-CO)3 AND ITS MONOSUBSTITUTED DERIVATIVES
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A kinetic investigation of carbon monoxide exchange processes in the parent tetranuclear cobalt carbonyl cluster, Co4(CO)12, and its monosubstituted analogues Co4(CO)11L (where L = phosphorus donor ligand) is reported. The Co4(CO)11L derivatives were concluded to be isostructural on the basis of (CO) infrared and 13C NMR spectroscopy aided by X-ray structural analysis of a representative species. The carbonyl-exchange reactions were monitored by infrared spectroscopy in the (CO) bridging region employing 13C-labeled carbon monoxide, which has allowed as well for a detailed vibrational analysis of the (CO) stretching vibrations for the bridging carbonyl ligands. The activation parameters for CO dissociation in Co4(CO)12 and Co4(CO)11P(OEt)3 were determined to be H* = 24.9 1.9 and 27.5 3.3 kcal/mol, and S* = 2.9 5.9 and 11.0 10.2 eu, respectively. Although there was no significant effect on the rate parameters for dissociative CO substitution in the tetranuclear cluster upon replacement of CO by a ligand of comparable spatial requirements (e.g., P(OMe)3 or P(OEt)3), dissociative CO loss was enhanced when the cluster substrate contained sterically more demanding ligands. A summary of comparative kinetic results in the stepwise phosphine or phosphite substitutional processes in Co4(CO)12 and Ir4(CO)12 species is included. 1982, American Chemical Society. All rights reserved.