The Ligand Effect

Ligands also play an important role in the catalyzed ORR. They not only serve as part of the active site, but also keep the metal in a stable form on the electrode surface. The substituents of the macrocycle ligand seem to have less effect in comparison with the type of macrocycle [23]. Phthalocyanine (Pc), tetrasulfophthalocyanine (TSP), tetracarboxyphthalocyanine (TcPc), polymeric phthalocyanine, tetraphenylporphyrin (TPP), tetramethoxyphenylporphyrin (TMPP), and dibenzotetraazaannulene (TAA) all can form complexes with Fe and Co. The majority of thus formed complexes have ORR catalytic activity. 

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The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

Selective Reactions on Modified Metal Surfaces the Ligand Effect. 201... 

Davies JC, Bonde J, Logadottir A, Nprskov JK, Chorkendorff I. 2005. The ligand effect CO desorption from PtRu catalysts. Euel Cells 4 429. 

A summary of the ligand effects of the palladium-catalyzed enantioselective hydrosilylation of styrene is given in Table 9. 

Thus, we are facing an impossible task to summarize the ligand effect for monodentate phosphines (and phosphorus ligands) and to give credit to the numerous contributions. For the sake of didactics we will present a few rules of thumb, which at this point will not be fully exemplified by literature data, but which will be supported when diphosphines are discussed in section 8.3. 

Applications of the model to Ru clusters have been reported namely by Toma etal. [78] and Keister etal. [79, 80], and, for example, for the single-electron oxidation of the 48-electron complexes of the type ]Ru3(/r-H)3(/r3-CX) (CO)9 L [, expression (20) is followed [79, 80], being derived from the general Lever s equation with inclusion of a Hammett term concerning the methylidyne X substituent. The observed value of 5 m3 (0.37) (vs. the unity expected for a monometallic Ru center) is indicative of an effective delocalization of the ligand effects over the three Ru atoms [79]. 

The Lever s model appears as the most general and attractive one. Since it assumes a complete additivity, with all the ligand effects treated separately and being independent from the metal, its application is rather general and simple. For instance, to estimate the redox potential of a particular complex, one simply has to add the contributions of all the ligands, SSl, and affect the total ligand effect by the effects associated to the redox couple (S m, /m), which... 

Those who prefer speculations on the ligand effects of alloying in hydrocarbon reactions may object that what is true for CO or H2 is not necessarily... 

It is of interest to note the ligand effect on the mechanisms of decomposition of several copper complexes of the type LCuin-CH3 ... 

The splittings of the d function manifold are conventionally dealt with by the formalism and techniques we call ligand field theory. The representation of the ligand effects by a one-electron potential is the central feature of the theory, conferring upon it great simplicity and hence utility. Recent studies (28) have shown that the approach is based on a much sounder footing than a brief acquaintance with quantum chemistry would indicate. 

The ability of heavy Group V ligands and transition metals to form stable L M—ER3 (E = P, As, Sb, Bi M = transition metal L = other ligands) is determined by the synergic interplay of their respective donor-acceptor properties, subtly modulated by steric influences. The electronic and steric factors determining the electron availability on the transition metal are determined by the oxidation state, coordination number, orbital geometry and the ligand effects of the other substituents in the coordination sphere. These factors will be discussed later. 

Pt-Sn surface. Even if hydrogen needs more than one transition metal atom in order to dissociate, this would only explain the large decrease in hydrogen adsorption if the possibility of spillover is largely reduced on the alloys. The ligand effect, to be discussed in Section IV, would have to be present for this assumption. 

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