Schrock-type alkylidyne complexes

Metal alkylidyne complexes undergo a variety of oxidation and reduction reactions as well as redox-induced transformations of the alkylidyne ligands. A method for the direct transformation of Fischer-type carbyne complexes into Schrock-type alkylidyne complexes was developed in our laboratory. Bromine oxidation of the /ra/7, -carbyne bromo tetracarbonyl complexes 49 of molybdenum and tungsten in the presence of dimethox-yethane affords the dme-stabilized alkylidyne tribromo metal complexes 50 [Eq. (42)] (81). For alkyl-substituted complexes (R = Me, CH2CMe3)... 

The preparation of high-valent, Schrock-type alkylidyne complexes containing poly(pyrazolyl)borate ligands may also be achieved by ligand substitution reactions. For example, the (dry) air-stable tungsten complexes Tp W( = CR)X2... 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

R.R. Schrock There are bands at ca. 1300 cm in the IR spectra of several alkylidyne complexes which might be assigned to W C stretching modes analogous to those observed by Fischer in complexes of the type (X)(C0)5W=CR (X = halide), but we have not done the appropriate labelling or Raman studies to confirm the assignments. 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

Complexes containing metal-carbon triple bonds in which the metal is in a relatively high formal oxidation state are frequently designated alkylidynes. Carbyne complexes are also sometimes classified as Fischer-type or Schrock-type, as in the case of carbene complexes. The distinction between these is discussed in Chapter 10. 

One route to a carbyne (alkylidyne) complex was reaction 23.40 equation 23.91 illustrates the initial method of Fischer. The abstraction of an ot-H atom from a Schrock-type carbene yields the corresponding carbyne complex (equation 23.92). 

A similar dichotomy of bonding models exists for other classes ofTM compounds where the chemical bond can be discussed either in terms of electron-sharing interactions TM-R or as donor-acceptor bond TM-L. Examples are TM compounds with carbenes CR2 or carbynes CR as ligands, which can be considered as either Fischer-type complexes or as Schrock-type alkylidenes and alkylidynes [31, 32). We want to emphasize that the two bonding models should be considered as sketches of two extreme situations, whereas the electronic situation of real molecules has components of both forms. The value of such dichotomic models lies in the fact that they establish an ordering scheme, which is very useful for describing the physical and chemical properties of molecules. 

ABSTRACT. The reaction between Schrock-type W alkylidyne complexes and inorganic oxides leads to several surface complexes, at least one of them bearing an alkylidene ligand. In the case of W(CCMe3)Np3/Nb205, the high catalytic activity seems to be more related to the reaction of a reduced W- species with the olefin via a tT-allyl mechanism than to the presence of the alkylidene ligand itself. 

Alkylidene and alkylidyne complexes are also referred to as carbene and carbyne complexes. Alkylidene complexes are of two types. The ones in which the metal is in a low oxidation state, like the chromium complex 2.37, are called Fischer carbenes. In contrast, complex 2.38, wha e the metal ion is in a high oxidation state, is referred to as a Schrock carbene. 

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