Novel Anionic Heterocyclic Thiolate Complexes of Group VIB Metal Carbonyls: Synthesis and Reactivity

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The syntheses and characterization of novel Group VIB metal carbonyl complexes containing heterocyclic thiolate ligands is described. The reactions of the photogenerated intermediate, M(CO)5THF (M = W, Mo, Cr), with the thiolate anions RS− in [(n‐Bu)4N][SR] [SR− = 2‐mercaptobenzimidazolyl (a), 2‐mercaptobenzothiazolyl (b), 2‐mercaptobenzoxazolyl (c)], afforded exclusively [(n‐Bu)4N][M(CO)5SR] in fairly good yields. Homo‐ and heterobimetallic complexes of the type [(n‐Bu)4N][M(CO)5(μ‐SR)M′(CO)5] (M, M′ = W, Mo, Cr), in which both the exocyclic S‐atom and the endocyclic N‐atom are involved in coordination to two metal centers, have been prepared by the reaction of [(n‐Bu)4N][M(CO)5SR] with M′(CO)5·THF at ambient temperatures. The methods of preparation and characterization of these complexes are described. CO‐substitution reactions in [(n‐Bu)4N][W(CO)5SR] [SR− = (b) and (c)] with tri(iso‐propyl)phosphite (L) afforded the complexes [(n‐Bu)4N][cis‐W(CO)4(SR){P(OPr‐i)3}] as the sole reaction product. The kinetics of CO dissociation from the pentacarbonyl anions under pseudo‐first‐order conditions confirm that these reactions are first‐order in substrate and independent of the entering group (L) concentration. The facile CO dissociation in these complexes is another example of neighboring‐group participation in assisting cis‐CO loss in octahedral metal carbonyls. From the kinetics data, the anionic ligands (b) and (c) are classified as cis‐CO labilizing groups. Rate constants and activation parameters for the CO‐substitution reactions in these complexes are reported.