Hydrocarbyl complexes group 11 metals

Imido dialkyl complexes, with tantalum, 5, 120 Imido-dicarbenes, with palladium, 8, 228-229 Imido groups, in Ru-Os mixed-metal clusters, 6, 1078 Imido hydrocarbyl complexes, with Re(VII), 5, 945 Imido ligands... 

Carbon monoxide insertion reactions in hydrocarbyl complexes of group VIII are particularly relevant because several industrial processes involving carbonylation of an organic substrate are catalyzed by metals belonging to this group. In the subsections that follow, reference to catalytic processes is made on each specific occasion. 

The Tpx ligands can mimic the coordination environment created by three imidazolyl groups from histidine residues, which is frequently found in the active sites of metalloenzymes. Higher valent bis(ix-oxo) species, [(Tpx)M( i-0)2M(Tpx)] via 0-0 cleavage of [(Tpx)M( x-r 2 r 2-02)M(Tpx)] intermediates, but also peroxo, hydroperoxo, and alkylperoxo species, active species undergoing oxidative C-C cleavage reaction, stable hydrocarbyl complexes, and dinuclear xenophilic complexes, [(Tpx)M-M L71], are all relevant to chemical and biological processes, most of which are associated with transition metal catalytic species. 

N-heterocyclic carbenes (NHC) are considered extremely effective hgands for homogeneous catalysis (Figure 6.2). These specific carbenes often lead to high efficiencies in metal-catalyzed reactions compared to traditional phosphines [49, 50). NHC complexes are usually considered to be very stable, due to their electronic properties and the unusually high bond dissociation energies (BDE) associated with NHCs [51]. Previous work has shown that Ni-hydrocarbyl complexes of NHCs readily decompose by reductive elimination to yield the 2-substituted imidazohum salts [52, 53). Later studies have shown that the reverse reaction, oxidative addition of imidazohum salts to zerovalent Group 10 metals, is feasible [53]. 

Simpler p-halide eliminations occur from late transition metal catalysts for olefin polymerization (Equations 10.25 and 10.26). Reactions of the cationic palladium-alkyl complexes occur in a similar fashion to the reactions of the cationic group 4 complexes, despite the softer nature of these species. In this case, propylene and the metal chloride are formed. Even a neutral nickel-hydrocarbyl complex (the salicaldimine complex in Equation 10.26) undergoes reactions with vinyl chloride that involve insertion followed by P-chloride elimination. 

Unactivated, unsaturated hydrocarbons are not routinely subject to attack by nucleophiles. Complexation of alkenes, alkynes, or arenes to a positively charged metal center may lead to activation of the hydrocarbyl group towards nucleophilic attack and, in particular, to addition of hydride ion. The effect of the [LnM] fragment on the reactivity of the unsaturated ligand is largely due to the transfer of electron density from the unsaturated hydrocarbyl to the metal center upon coordination, a process that is most effective when the metal center is positively charged. Figure 10.5 ... 

Insertion of carbon monoxide and alkenes into metal-carbon bonds is one of the most important reaction steps in homogeneous catalysis. It has been found for insertion processes of platinum [16] that the relative positions of the hydrocarbyl group and the unsaturated fragment must be cis in the reacting complex [17], The second issue concerns the stereochemical course of the reaction, insertion versus migration as discussed in Chapter 2.2. 

Neutral, cP group 4 metal alkyl complexes of general form CpjMCR) (R = H, hydrocarbyl) or Cp2M(R)(X) (X = anionic, two-electron donor)... 

Instead of the free acid it is sometimes advantageous to use a metal salt (e. g., of Cu of Ni") to introduce the anions. Preformed complexes of the type L2 dX2 [18] and L2Pd(R)X [19] where L2 represents a chelating ligand, X a weakly coordinating anion, and R a hydrocarbyl group (e. g., methyl) have also been tested as catalysts. The results are, generally, very similar to those obtained with catalysts prepared in situ. 

Transition and nontransition elements c-bonded to a carbon-containing residue (an alkyl or an aryl group), which constitute the class of hydrocarbyl metal complexes, may undergo insertion reactions. The term insertion should be clarified, since without further qualification, it simply refers to the stoichiometric result of the reaction, no mechanistic details (see 11.3.2.1.1) being involved. 

As stated earlier, (11.3.1), the multiple insertion of carbon monoxide into the same metal-hydrocarbyl bond is a rather elusive reaction. On the other hand, multiple insertion of isocyanide has been reported for nickel(II). For example, when the nickelfO) derivative Ni(t-BuNC)4 was treated with Mel in hexane at RT, consecutive insertion of three RNC groups was observed to give the product of reaction (e), as a consequence of a primary oxidative addition of the alkyl iodide to the nickel(O) complex. It is interesting that one of the two terminal fragments of the five-membered metallacycle is reminiscent of an arrangement of the first insertion product. 

D. Jeremic, D.W. Stephan, Hydrocarbyl phosphinimine/ cyclopentadienyl complexes of group IV metals and preparation thereof, CA 2282070 of 09 Dec 2008 (filed 10 Sept 1999) ... 

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