Ligands as Lewis bases

This brings up the mechanics of electron counting. The convention that we shall use is to treat all ligands as Lewis bases. Listed in 16.7 arc some typical two-clcctron a donor groups. In 16.8 are listed some two-electron a donors which also have one... 

It is useflil to show the valence bond representations of the complexes [CoFe] and [Co(NH3)6], which can then be compared with representations from the crystal field and molecular orbital theories to be discussed later. First, we must know from experiment that [CoF ] contains four unpaired electrons, whereas [Co(NH3)g] has all of its electrons paired. Each of the ligands, as Lewis bases, contributes a pair of electrons to form a coordinate covalent bond. The valence bond theory designations of the electronic structures are shown in Figure 2.7. The bonding is described as being covalent. Appropriate combinations of metal atomic orbitals are blended together to give a new set of orbitals, called hybrid orbitals. 

Organo-ligands which can pick up a proton should also be able to act as Lewis bases towards metal ions, but only two cases have so far been reported. Ethinyl- and vinylcobalamin both show reversible equilibria with Ag(I) ions, which have been ascribed to equilibria such as... 

Removing electrons from a metal atom always generates vacant valence orbitals. As described in Chapter 20, many transition metal cations form complexes with ligands in aqueous solution, hi these complexes, the ligands act as Lewis bases, donating pairs of electrons to form metal-ligand bonds. The metal cation accepts these electrons, so it acts as a Lewis acid. Metal cations from the p block also act as Lewis acids. For example, Pb ((2 g) forms a Lewis acid-base adduct with four CN anions, each of which donates a pair of electrons Pb ((2 ( ) + 4 CN ((2 q) -> [Pb (CN)4] (a g)... 

The NMR spectra of the two-coordinate stannylenes in solution show values of Sn ranging from about 1150 (e.g., in ArSnl) to 3750 (in (Ar3Sn)Sn ), with a large anisotropy. The stannylenes behave as Lewis acids, for example, in the three- or four-coordinate complexes (e.g., 78, 79, and 80), which are formed when the molecule carries an intramolecular ligand, and as Lewis bases, particularly in complexing to transition metals (e.g., 81, 82, and 83). The dimerization of stannylenes to give distannenes can be regarded as a result of this amphoteric character (Equation (179)). 

The rational design of chelating agents as antidotes requires a careful consideration of acid-base chemistry. Metal ions are Lewis acids, while the chelating agents or ligands are Lewis bases. The concepts of hardness and softness may be used to describe systematically the interaction between them. A hard metal cation is one that retains its... 

The ligand of a metal complex and the solvent molecule compete with each other, as Lewis bases, to interact with the central metal ion. At the same time, the metal ion and the solvent molecule compete with each other, as Lewis acids, to interact with the ligand. Thus, the behavior of a metal complex is easily influenced by the Lewis acid-base properties of solvents. In an aprotic solvent, which is of weak acid-... 

Evidence was presented that cobalt precursors under the reaction conditions are transformed into cobalt carbonyls.31 Additives such as Lewis bases accelerate the formation of the catalyst.11 [CoH(CO)4] the key catalytic species was shown by infrared (IR) spectroscopy to be formed under hydroformylation conditions32 and was isolated in the reaction of [Co(CO)4]2 and hydrogen.33 [CoH(CO)4] dissociates carbon monoxide to create [CoH(CO)3] [Eq. (7.2)], which is capable of olefin com-plexation because of a ligand vacancy ... 

The molecules or ions (typically, four or six) that are the Lewis bases attached to a central metal atom or ion in a d-metal complex are known as ligands. One example is the hexacyanoferrate(Il) ion, [Fc(CN)J4, in which the Lewis acid Fe2+ forms bonds by sharing electron pairs provided by the CN ions. An example of a neutral complex is Ni(CO)4, in which the Ni atom acts as the Lewis acid and the ligands, the CO molecules, act as Lewis bases. 

The molecules or ions that surround the central metal ion in a coordination compound are called ligands, and the atoms that are attached directly to the metal are called ligand donor atoms. In cisplatin, for example, the ligands are NH3 and Cl-, and the ligand donor atoms are N and Cl. The formation of a coordination compound is a Lewis acid-base interaction (Section 15.16) in which the ligands act as Lewis bases (electron-pair donors) and the central metal ion behaves as a Lewis acid (an electron-pair acceptor). 

Shibasaki et al. have reported an asymmetric nitroaldol reaction catalyzed by chiral lanthanum alkoxide 18 to produce an optically active 2-hydroxy-1-nitroalkane with moderate-to-high enantiomeric excesses (Scheme 8B1.10) [27]. Apparently this novel catalyst acts as Lewis base. The proposed reaction mechanism is shown in Scheme 8B1.11, where the first step of the reaction is the ligand exchange between binaphthol and nitromethane. This reaction is probably the first successful example of the catalytic asymmetric reaction promoted by a Lew i s base metal catalyst. Future application of this methodology is quite promising. 

As has already been discussed, ligands function as Lewis bases when they donate pairs of electrons to a metal ion, which behaves as a Lewis acid, and the coordinate bonds are the result of electron pair donation and acceptance. When NH3 is added to a solution containing [Cu(H20)4]2+, the color changes to a dark blue because of the formation of [Cu(NH3)4]2+ as a result of the substitution of NH3 molecules for H20 molecules. In fact, the most frequently encountered type of reaction of complexes is the substitution of one ligand for another. 

Each of these cases can be considered as a disproportionation that produces a positive metal ion and a metal ion in a negative oxidation state. The carbonyl ligands will be bound to the softer metal species, the metal anion, and the nitrogen donor ligands (hard Lewis bases) will be bound to the harder metal species, the cation. These disproportionation reactions are quite useful in the preparation of a variety of carbonylate complexes. For example, the [Ni2(CO)6]2 and [Co(CO)4] ions can be prepared by the reactions... 

Efforts to explore and categorize the reaction patterns of triply-bonded dimers of molybdenum and tungsten of the types M2Lg (L = R, NR2 or OR) and (r -CsHs )2M2(C0K, i.e., those of classes (C) and (D), have been pursued 8-13) Reactions which may be viewed as examples of ligand substitution, Lewis base association,... 

In addition to this Lewis-acid behavior, 16- and 18-electron metal complexes can act as Lewis bases, i.e., they possess accessible electron pairs. This Lewis basic character depends strongly on the donor and acceptor strengths of the ligands and is very pronounced in complexes of strong donors such as trialkylphosphines. For example, whereas CpCo(CO)2 shows little basic character and little tendency to react with electrophiles such as CH3I, CpCo(PMe3)2 is a strong metallic base .10 Such compounds are particularly reactive towards oxidative addition reactions ... 

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