Pi Ligands

In Chapter 5 we consider interactions between ligand n systems and metals, beginning with the simplest of the linear systems—ethylene—and then proceeding with more complex linear and cyclic systems. During this discussion, we pay special attention to the classic example of the compound ferrocene. 

Other metal orbitals with suitable symmetry 

Allyl complexes (or complexes of substituted allyls) are intermediates in many reactions—some of which take advantage of the capacity of this ligand to function 

Coriani, A. Haaland, T. Helgaker, and P. J0rgensen, ChemPhysChem, 2007, 7, 245, and references cited therein. 

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Tab. 1 General relationship between the electrochemical Pi ligand parameter and the electronic properties of the ligand (L) and its coordination M —L bond...

Hence, the Pi ligand parameter reflects, in an overall way, the combined a- and Tt -electronic properties of the coordination M—L bond. It is noteworthy to mention that it relates to the variation of the free-energy difference of the redox processes (consider the known expression AG = —nFE, in which n is the number of electrons transferred and F is the Faraday constant). It has analogies with the Hammett Up constant [11, 12] defined as og[Kx/Kh), that is, log Kx - log K, in which Kx and ATh are the acidic constants of the p-substituted benzoic acid HOOCCg H4X-4 and of benzoic acid itself, respectively [13] (consider also the known relationship AG = —RT nK). 

I 3 Redox Potential-structure Relationships in Coordination Compounds Tab. 2 Values of the Pi ligand parameter for ligand L ... 

Tab. 3 Values of the Pi ligand paranneter for metallo-cyanide C=N —ML i and other cyano-adduct ligands ...

The dependence of the Pi ligand parameter on the type of binding metal center is a limitation of the Pickett s model, but a similar difficulty is encountered for the Ei Lever parameter (see in the following) for which the need for corrections has been recognized for ligands such as CO and CNR. Such a type of limitation is inherent to any additive model that tries to separate the effects of ligands and metal centers on the redox potential, and add them as independent components, in contrast with the situation of the real molecule in which those effects are mutually dependent. 

Hydrolysis of E-site GTP during or shortly after addition of tubulin to the microtubule ends and the gain and loss of a short region of GTP-(or GDP-Pi)-liganded tubulin at the extreme ends of the microtubules that stabilize the microtubule tips are believed responsible for dynamic instability. The tubulin dimer has intrinsic GTPase activity, with a rate that is relatively slow when tubulin is in solution (21). However, hydrolysis is triggered when the 3-subunit of an incoming tubulin dimer with bound GTP docks at the end of an exposed a-subunit... 

Trends in the wCo chemical shifts of a wide variety of organocobalt complexes have been usefully interpreted in terms of structure-activity relationships in homogeneous catalysis (42) and in terms of the influence of various pi-ligands on structure and... 

A variation on the theme of metallocenes and related sandwich compounds is provided by the inverse sandwich in Figure 13.34, with calcium(I) ions on the outside and the cyclic pi ligand 1,3,5-triphenylbenzene in between. This compound was most efficiently prepared by reacting 1,3,5-triphenylbenzene with activated calcium in THF solvent using catalytic amounts of l-bromo-2,4,6-triphenylbenzene. Although the product of this reaction is highly sensitive to moisture and air and is pyrophoric, it represents a rare example of a +1 oxidation state among the alkaline earths. 

The same type of behavior can be found for nucleophilic attack of coordinated olefins and arenes. The C=C group of (cod)PdCl2 is attacked only by basic MeOH, while [(cod)Pd(P/ 3)Cl] is rapidly attacked even in neutral MeOH. " Hydride addition to coordinated pi-ligands tends to be controlled by the charge on the metal, for example ... 

The halogenated phenol, e.g., pentafluorophenol, CsFsOH, pmnits the substitution of a portiOTi of the expcmsive MAO with inexpensive Al(alkyl)3. This co-catalyst is most frequently used with an organometallics having >1 PI ligand. The copolymerization of C2 with Ca.jo may be carried out at T = 30-280 °C and P = 14-22 MPa in a dual-reactor system... 

Tab. 6 Val ues of the Pi ligand parameter for cyanamides at different metal centers...

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