Tungsten complexes, metal exchange reactions

Further studies of the reaction of molybdenum and tungsten carbonyl complexes with Lewis acids have been reported and several interesting new adducts characterized. [Mg(py)4][(7r-Cp)Mo(CO)3]2 has been obtained by a metal-exchange reaction from [Hg Mo(7t-Cp)(CO)3 2] in THF, followed by recrystallization from pyridine. X-Ray crystallographic studies have shown the compound to possess the units (67), the dimensions of which suggest that an... 

In addition to the bimetallic complexes of rhenium and alkaline metals formed as byproducts in the exchange reactions of rhenium halids with alkali alkoxides (such as, for example, LiReO(OPr )5 xLiCl(THF)2 [519]) there has been recently prepared a number ofbimetallic complexes ofrhenium and molybdenum, rhenium and tungsten, and rhenium and niobium [904, 1451]. The latter are formed either due to the formation of a metal-metal bond, arising due to combination of a free electron pair on rhenium (V) and a vacant orbital of molybdenum (VI) atom or via insertion of molybdenum or tungsten atoms into the molecular structure characteristic of rhenium (V and VI) oxoalkox-ides. The formation of the compounds with variable composition becomes possible in the latter case. 

Arene complexes are usually prepared by the displacement of more weakly bound, monodentate ligands, by oxidation or evolution of CO ligands, or by reducing a metal from a higher oxidation state in the presence of an arene. These methods have been reviewed. - In some cases, one arene will displace another. For example, hexamethylbenzene replaces p-diisopropyl benzene in the tungsten complex of Equation 2.13. This reaction is imdoubt-edly controlled by steric effects. The mechanisms of these arene exchange reactions have been clarified by Traylor and are discussed in Section 4.55. 

The reaction of anionic metal complexes 162 with a-chloro ketones, esters, or tertiary amides occurring under chloro-metal exchange is an appropriate method for the preparation of molybdenum and tungsten enolates. The reaction fails with the corresponding a-bromo compounds. In order to obtain tungsten enolates, which are substituted in a-position, a-methanesulfonyloxy-substituted esters are suitable starting materials. The tungsten and molybdenum complexes 163 were fully characterized as C-bound // -enolates (Scheme 2.47) [162]. 

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