Function of the Ligand

Many of the properties of metal ions in aiding or discouraging reactions are beautifully illustrated in the work on the chemical synthesis of corrins. Thus, it is appropriate to conclude this chapter by a relevant quotation from Eschenmoser  

The role of transition metals in the chemical synthesis of corrins is more than just adding a touch of inorganic elegance to organic synthesis- leave elegance to tailors and cobblers , a physicist once said — it is a vital role in the sense that perhaps no synthetic corrin would as yet exist without recourse to metal templates. Beside the purely topological function of arranging the proximity of reaction centres, metal ions have served this purpose as follows  

Lewis Acid Catalysis and the Reactions of Coordinated Ligands in B35, Vol. 6, Chap. 61.4. 

Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, NY, 1969, p. 8. 

Hendry and A. M. Sargeson, Inorg. Chem. 25, 865 (1986). The Co—NHj entity is generated in a rapid preequilibrium involving OH attack on the Co —NH3 bond. 

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By far the most studied pyrazole-containing ligand is the symmetrical monoanionic tripod hydridotrispyrazolylborate, Tp (3). Substituents introduced at each of the 3-, 4- or 5-positions (3,5-Me2Tp108, 3-z Pr,4-BrTp109) have led to a number of sterically encumbered threefold symmetric complexes and some remarkable Co coordination chemistry has ensued. Hydride substitution at the B atom is also possible by organic groups such as /z-bromophcnyl, which enables further functionalization of the ligand.110... 

When the combination of the two wave functions of the ligands and the pz orbital of the metal is made, the signs of the lobes on the pz orbitals require the negative lobes on the "bottom" ligand wave function be directed upward as shown in Figure 21.17. The combination of the dyz orbital can be... 

Vincente-Perez, S. Durand, J. S. Alvarez, M. D. 1992. Limitations of complexes logarithmic diagrams as a function of the ligand concentration-diagrams of conditioned variable. An. Quim. 88(7-8) 683-688. 

Fig. 5 Regions of stability of either one or the other spin state as a function of the ligand-field strength. The region of spin crossover compounds is indicated by the shaded area. For the calculation, the values of the Racah parameters were taken to be 75% of the free ion values. This corresponds to a typical reduction for iron(II) coordination compounds of the type under consideration... 

For example, a proline-based chiral ligand was attached to a vinyl-substituted monomer (Fig. 42.15) by reacting vinylbenzoyl chloride with the amine functionality of the ligand [106]. As mentioned previously, the apolar Merrifield resin as a support is not swollen in polar solvents. Hence, in order to match the polarity of the resin with that of the typically used substrates in enantioselective hydrogenation, the functionalized monomer was copolymerized with polar units of methacrylic acid 2-hydroxyethyl ester. 

Qualitative completeness of x means that, if a is not the operator for a racemic mixture, then /( Mq), considered as a function of the ligands, does not vanish identically. 

Figure 9.3. Schematic comparison of the direct pair potential U(R) and the potential of average force as a function of the ligand-ligand separation.

The second case is when the reservoir is a solution of the ligand in a solvent, say water. The form of the BI is the same, except for areinterpretation of the quantity G . In Eq. (D.7), G is an integral over the pair correlation function of the ligands in vacuum, i.e., in Eq. (D.9) is the ligand-ligand pair potential. In the case... 

Whenever there are two or more identical atoms linked to a central atom, their wave functions must be combined in such a way as to demonstrate their indistinguishability. This is achieved by making linear combinations of the wave functions of the ligand atoms. For the two Is orbitals of the hydrogen atoms in the water molecule their wave functions may be combined to give ... 

Thus far we have seen that Schiff base ligands tend to promote six-coordination, but an unusual reaction has been found to occur between TiCl2(salen) and borohydride. No reduction at titanium is seen, but rather, addition of BtLr to the imino function of the ligand occurs to give a dimeric seven-coordinate titanium(IV) compound, (24), containing amine-boranes as... 

The valence orbitals taken for a molecular-orbital calculation of a transition metal complex are the nd, (n + l)s, and (n + l)p metal orbitals and appropriate a and n functions of the ligands. Many of these valence orbitals are not individually basis functions for an irreducible representation in the symmetry under consideration. Symmetry basis functions transforming properly must be constructed, by methods analogous to those used throughout this volume. The results for a number of important symmetries are tabulated in this volume in various places, as follows ... 

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