CARBON-MONOXIDE LIGAND SUBSTITUTIONAL PROCESSES INVOLVING ANIONIC GROUP-6 METAL CARBOXYLATES AND THEIR RELEVANCE TO DECARBOXYLATION MECHANISMS

Mechanistic aspects of ligand substitution reactions of [W(CO)5O2CCH3]− complexes with phosphorus donor ligands have been investigated. Kinetic measurements show the process to be first order in metal substrate and nearly zero order in incoming ligand concentration. The activation parameters, determined for the reaction of [W(CO)5O2CCH3]− with P(OCH3)3 as entering ligand to afford cis-[W(CO)4(P(OCH3)3)O2CCH3]− + CO, are ΔH* = 23.3 ± 0.8 kcal mol−1 and ΔS* = 5.4 ● 2.6 eu. This facile carbonyl displacement reaction is ascribed, in part, to an intramolecular ligand-assisted dissociation of CO, where the distal oxygen atom of the acetate moiety interacts with the cis CO ligands to effect their selective loss. Consistent with this description, the [W(CO)4(η2-O2CCH3)]– complex has been synthesized and characterized by infrared and 13C NMR spectroscopies and shown to undergo rapid reactions with incoming ligands (PR3 and CO) to yield cis-[W(CO)4(L)O2CCH3]– compounds. The relevance of CO dissociation and the resultant formation of a chelated acetate ligand to the decarboxylation reactions of the closely related formate derivatives is discussed. © 1990, American Chemical Society. All rights reserved.