From metal and ligand

From metal and ligand. When the metal in the starting material is already in a formal oxidation state suitable for the formation of the desired product, direct ligand substitution can occur, without the necessity to include a reducing agent in the reaction mixture. As a very simple example of this the direct formation of the carbonyls of iron and of nickel from carbon monoxide and the metal may be cited  

It is uncommon, however, for ligands to react directly with metals to yield complexes, probably as the products woidd be thermally unstable under the conditions, e.g. of high temperature, under which the reactions would occur at a reasonable rate. 

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Table A6.1. Stability Constants" for Formation of Complexes and Solids from Metals and Ligands. Compilation by Morel and Hering (1993).

FIGURE 2. Hel/Hell UPS of AuMe3(PPh2Me) showing the band manifold for ionizations from metal and ligand orbitals. Reprinted with permission from Reference 8b. Copyright 1983 American Chemical Society... 

The most commonly occurring coordination numbers for transition elements are 4 and 6, but all values from 2 to 9 are known and a few examples of even higher ones have been established. The more important factors determining the most favourable coordination number for a particular metal and ligand are summarized below. However it is important to realize that, with so many factors involved, it is not difficult to provide facile explanations of varying degrees of plausibility for most experimental observations, and it is therefore prudent to treat such explanations with caution. 

Fabiato, A. (1988). Computer programs for calculating total from specified free or free from specified total ionic concentrations in aqueous solutions containing multiple metals and ligands. Method. Enzymol. 157 378-417. 

Figure 2.38 The effect of varying the relative energies of the metal and ligand orbitals upon the final molecular orbital scheme for a dimeric rhodium carboxylate. (Reprinted from Coord. Chem. Rev., 50, 109, 1983, with kind permission from Elsevier Science S.A., P.O. Box 564,...

Bonding orbitals in a metal complex may be thought of as molecular orhitals built from appropriate metal and ligand functions. In the case of an M-L o bond orbital, i/ , for example, we write... 

They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

The ligands are referred to as ir acceptors because of their receiving electron density donated from the metal to 7rg orbitals. Back donation results in increasing the bond order between the metal and ligand, so it results in additional bonding. 

As a further illustration of the dependence of n i 7t pi-backbonding interactions on metal and ligand character, we may compare simple NiL complexes of nickel with carbonyl (CO), cyanide (CN-), and isocyanide (NC-) ligands, as shown in Fig. 4.41. This figure shows that the nNi 7rL pi-backbonding interaction decreases appreciably (from 28.5 kcal mol-1 in NiCO to 6.3 kcalmol-1 in NiNC-, estimated by second-order perturbation theory) as the polarity of the 7Tl acceptor shifts unfavorably away from the metal donor orbital. The interaction in NiCO is stronger than that in NiCN- partially due to the shorter Ni—C distance in the... 

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