, Volume 28, Issue 2, pp 193-198

Reactivity of tungsten(0) and molybdenum(0) pentacarbonyl thiobenzoate anions: thiobenzoate as a cis-CO labilizing ligand

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Abstract

The [Et4N][M(CO)5SCOPh] complexes (1a, M = Mo; 2a, M = W) have been prepared at ambient temperatures by reacting the photogenerated M(CO)5 THF intermediate with [Et4N][SCOPh] in THF. Kinetic studies of the reactions of the anions [M(CO)5SCOPh] with the tri(iso-propyl)phosphite (L) ligand under pseudo-first-order conditions indicate that these reactions are first-order in substrate and are independent of the P(OPr-i)3 concentration. It is thus envisaged that these CO substitutions proceed via a mechanism which involves initial cis-M—CO bond-breaking, followed by fast attack of the incoming nucleophile on the resulting intermediate to give [cis-M(CO)4{P(O-Pri)3}SCOPh]. This facile displacement of cis-CO indicates the labilizing nature of the thiobenzoate ligand, most probably by virtue of distal oxygen atom participation. Activation parameters for the reactions are: [M(CO)5SCOPh] + L → cis-[M(CO)4(L)SCOPh] + CO M = Mo, ΔH = 24.6(2) kcal mol−1, ΔS = 8.2(6) eu; M = W, ΔH = 28.4(2) kcal mol−1, ΔS = 11.3(5) eu. Kinetic data and the mechanism of these ligand-substitutions are discussed.