STEREOCHEMICAL NONRIGIDITY IN 6-COORDINATE GROUP-6B METAL-CARBONYL DERIVATIVES VIA A NONDISSOCIATIVE PATHWAY
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The cis-M(CO)4(13CO)PR3 trans-M(CO)4(13CO)PR3(M = Cr, R = Et; M = W, R = OMe, Me, Et, i-Pr) isomerization reactions have been studied kinetically in cyclohexane-d12by 13C NMR spectroscopy. These isomerization reactions were shown to proceed via an intramolecular process, based on the lack of 13CO incorporation into the M(CO)5PR3derivatives or formation of M(CO)5(13CO) under reaction conditions of ligand rearrangement. The cis trans CO scrambling process in the tungsten pentacarbonyl derivatives displays a dependence on the steric nature of the phosphorus ligand, with the rate decreasing PMe3> PEt3> P-i-Pr3. The origin of this steric effect is seen in the activation parameters (H = 9.2 kcal mol-1and S =-54.9 eu), which are indicative of gross reorganization in the transition state with little metal-ligand bond weakening. On the other hand, activation parameters for carbon monoxide ligand isomerization in Cr(CO)4(13CO)PEt3(H = 26.6 kcal mol-1and S = 1.80 eu) are consistent with considerable metal-ligand bond breaking in the transition state with little reorganization taking place. 1984, American Chemical Society. All rights reserved.
