7r-Ligand

The Lever s model has also been extended to sandwich and half-sandwich complexes with 7T-cyclopentadienyl or 7r-arene ligands [66, 67, 69]. The parameter for the 7r-ligands has been defined [69] on the basis of the low spin Fe hH redox couple, by Eqs. (21) or (22), for homolep-tic sandwich [Fe( 7r-L)2] or mixed sandwich ]Fe( 7r-Li)( 7r-L2)] complexes, respectively. 

Some interesting reactions have recently been found to occur on heating cycloheptatriene with some organosiliconruthenium carbonyls. An unexpected feature of these reactions is the migration of trimethyl-silyl groups from the metal to the 7r-ligand (R = Me) (17, 74) ... 

Metal Complexes of 7r-Ligands Containing Organosilicon Groups, 15, 113... 

A plausible reaction mechanism for such couplings is presented in Figure 13.7 for the specific example of the transformation of Figure 13.6. We do not specify the number n and the nature of the ligands L of the intermediate Ni complexes in Figure 13.7. Little is known about either one. Also, it is quite possible that more than one elementary reaction is involved in some of the steps 1-5. In any case, these five steps are certainly involved in the overall reaction. In step 1, the aryl bromide enters the coordination sphere of the Ni(0) compound as a 7r ligand. At least one metal coordination site must be vacated before an oxidative addition of the aryl bromide to the Ni atom may occur in step 2. The nickel inserts into the Csp2—Br bond, and its oxidation... 

The only complex in Table I with identical ligands in the two positions cis to the alkyne is the [(775-C9H7)MoL2(MeC=CMe]+ cation with L = PMe3 (72). Here the 2-butyne is parallel to one Mo—L vector and perpendicular to one Mo—L vector (Fig. 11). The 7r-ligand properties of the PMe3 ligands are probably not sufficiently dominant to create a substantial alkyne preference between the dir orbital combinations which are available. 

The reactions described in Section II dealt with reactions of binary nickel(0)-olefin complexes with AmR, AmH, and AmPR2 and are characterized by the fact that the charge of alkali metal atoms is transferred to nickel and the respective 7r-ligands via C, H, or P atoms. That the charge from alkali metal atoms can also be directly transferred to nickel(O) -jr-lig-and systems is demonstrated in the syntheses (14, 31-33), structures (14, 34), and NMR data (18,35) of the dilithium-nickel-olefin complexes now described. 

Thus in 7r-ligand substitution a change in metal coordination number rather than direction of electron density shift in the transition state is quite significant. 

Compounds with -Ligands without Anionic 7r-Ligands 325... 

Structures 23.1a and 23.1b show two representations of an [t -C5H5] (cyclopentadienyl, Cp ) ligand. For clarity in the diagrams in this chapter, we adopt 23.1b and similar representations for 7r-ligands such as r -C3H5 and... 

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