Alkyl olefin complexes, structures

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

There appear to be no known structures of 7>(P)(H)(alkyl) complexes. Table 1 tabulates rr(P)(H)(olefin) complexes. Even for these the tabulation is very small and the results are unsatisfactory. There are only two complexes involving a mono-olefin, 1 and 2. In neither of these structures was the position of the hydride ligand located. But on the basis of the stereochemistry it is clear that H is cis to olefin in 2. In fact, compound 2 represents the best (and only) model for the generally proposed intermediate. That it contains an activated olefin, tetracyanoethylene, and Ir rather than Rh may be the reason for its stability. 

We saw in 15.31 and the discussion around it that the orbitals of CH2 were topologically like those in ethylene. Each has a donor orbital of 0 symmetry and a low-lying acceptor orbital. The electronic structure of an (alkyl)(olefin)MCp2 complex is then similar to the Cp2M (carbene) ones which we have just treated. 20.47 shows the Io -tt interaction which is analogous to the tt interaction in 20.46... 

Complex formation with flic substrate is the key stage of many catalytic processes. The formation of the following types of organometallic complexes is most typical in catalysis alkyl 7i-complexes, carbene complexes, n-complexes of substrates with the saturated bond (olefin, acetylene and allyl, complexes with carbon oxides), hydrazine complexes, and complexes with molecular oxygen and nitrogen. The structure of a ruthenium complex with CO2 obtained on the basis of an ab initio study is presented in Fig. 17.4. 

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