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2.5.1.1 Method 1: Ligand Substitution Reactions of Hexacarbonylchromium(0) or Hexacar­bonylmolybdenum(0) with Trienes

DOI: 10.1055/sos-SD-002-00251

Theopold, K. H.; Mommertz, A.; Salisbury, B. A.Science of Synthesis, (20032229.

The homoleptic hexacarbonyl complexes of the group 6 metals are readily available and convenient starting materials for this subclass. The relatively low mean MCO bond energies, especially of the chromium and molybdenum derivatives [Cr(CO)6 26 kcalmol1, Mo(CO)6 36 kcalmol1, W(CO)6 43 kcalmol1] allow for thermal expulsion of carbon monoxide,[‌7‌] followed by trapping of the coordinatively unsaturated metal fragments with various ligands, including trienes (Scheme 1). The reaction is typically carried out in pure triene or in a high-boiling solvent. This method was first used by Wilkinson and co-workers in 1958 to prepare the first cycloheptatriene complex, namely tricarbonyl(η6-cycloheptatriene)molybdenum(0) (1) (Scheme 1).[‌8‌] The higher bond strength of hexacarbonyltungsten(0) effectively limits the scope of this procedure to the preparation of the chromium and molybdenum complexes, while tungsten complexes are best prepared via mixed nitrile/carbonyl compounds (see Section 2.5.1.1.1). Complexes of unsubstituted as well as monosubstituted (e.g., 3) and polysubstituted cycloheptatriene ligands (e.g., 2AC) are accessible by this method (Scheme 1).[‌9‌,‌10‌] Tricarbonyl(η6-cycloocta-1,3,5-triene)chromium(0) has also been prepared this way.[‌11‌] The intervention of coordinatively unsaturated intermediates suggests incompatibility with substituents that are better ligands (e.g., phosphines) or that may undergo oxidative addition to the zerovalent metal (e.g., alkyl halides).

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