Other Coordinated Ligands

Decomposition of the copper complex with HjS in acid leads to allothreonine and (in about 2-fold excess) threonine. Following the pioneering work of Akabori et this reaction has been extended to a variety of metals and aldehydes and to complexes such as A- and A-Co(en)2gly with intriguing stereoselectivity behavior (optically-active allothreonine and threonine products).  

The racemization rate constant for L — Ala coordinated to Co(III) is much larger than for free L — Ala. In addition, the larger the positive charge on the complex, the faster is the rate of racemization. These observations give credance to the idea that racemization (and exchange) takes place by an initial abstraction of an a-H by base, resulting in a carbanion.  

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A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

A number of metal carbonyls undergo disproportionation reactions in the presence of other coordinating ligands. For example, in the presence of amines, Fe(CO)5 reacts as follows ... 

As a result of the strong tendency toward 3c/4e hypervalent bonding, each M—L coordinative bond of a normal-valent ML transition-metal complex will be susceptible to successive cu-additions by other coordinative ligands L cf. (3.212a) ... 

A detailed stopped-flow kinetics investigation (85) of this reaction (Por = TmTP and OEP) demonstrated that the mechanism occurs in two stages. The first was quite fast and was suppressed by the presence of excess CO and other coordinating ligands. Time resolved infrared... 

Nephelauxetic or other coordinated ligand effects which delocalize electron density may lead to a numerically smaller value of a than found for the free ions. 

Although it is well known that coordinated carbon monoxide in metal carbonyls may be replaced quite easily by other coordinating ligands, only a few studies have been made of the exchange equilibria resulting from the... 

Transition metal complexes of both of these classes of carbene had been prepared previously by modification of other coordinated ligands (Figure 5.7c). 

Ni(H20)6] + (73) is the main cation formed when Ni is dissolved in aqueous solution in the absence of other coordinating ligands a second hydration sphere of some 15 water molecules has been proposed. In addition, most simple salts written as NiX2-xH20 actually contain the hexaaqua ion [Ni(H20)6] +. Under alkaline conditions, nF-hydroxy... 

E. Oxygen Transfer from Nitrosyl Groups to Other Coordinated Ligands... 

Other examples of the two types of sites behaving differently were provided by the results of poisoning experiments. Polar or other coordinating ligands are usually strong poisons for chromium oxide catalysts,... 

This chapter is concerned with a quantum chemical description of sulphonic acids and their derivatives. The characteristic feature of this class of compounds is the four coordination of the central sulphur atom, including the sulphonyl fragment > S02, where one of the other coordinating ligands is usually an acidic (—OH) or acid derivative group. We have taken the definition of acid derivatives here in a broad sense. Therefore, some of the material in this chapter is relevant to, and refers to, the previous volume in this series on sulphones1. 

New Species from Reactions of Halocarbonylmetallate Anions. The basic idea for this aspect of the work was that reactive metal halide fragments might be generated from the halocarbonylmetallate anions under conditions where loss of the CO ligands is promoted in the absence of other coordinating ligands. For example, dinuclear or cluster species might be obtained via thermal decomposition of the M(CO)4X anions as in the scheme ... 

The specific acceleration of departure of anions also requires that attack of anions will be accelerated by complexation of the nucleophile with a metal ion. Metal-ion-catalysed enolization reactions of 3-ketophosphonates has been examined in order to investigate the interactions of metals and phosphates. In these studies (Kluger et ai, 1984b) it was found that the metal ion serves to provide the catalytic functions described previously. In addition, from the observed change in steric effects on the reaction in the presence of metals, it appears that the coordinated base can react through a mechanism whereby it serves as a relay for reaction by other coordinated ligands. 

Commonly utilized starting transition metal materials are oxides, halides, salts of fatty acids, complexes containing sulfur, etc. The role of the reducing agents is to lower the oxidation state of a metal and not to reduce the compound to an active metallic phase because this would, in most cases, lead to the inhibition of the reaction as shown by thermodynamic calculations. This point is also indicated by the formation of carbonyl derivatives possessing CO and other coordinating ligands as intermediates. 

Other examples of nonterminal nucleophilic attack on cyclic dienylium systems are known. The position of attack depends on ring size, the transition metal involved, and the other coordinating ligands. These effects have been discussed (Edwards et al., 1974 Deeming et al., 1974). The reason for nucleophilic attack at the 3-position of the dienylium system in Eq. (121) may be due to initial coordination of triphenylphosphine to the coordinatively unsaturated Mo atom. The resulting tricarbonyl(triphenylphosphine)Mo complex could then undergo transfer of triphenylphosphine from Mo or attack by a second molecule of triphenylphosphine at the 3-position of the dienylium system. 

In a number of instances the dithiocarbamate acts as a monodentate ligand B (178, 179, 230-245). This often results when the demands of the other coordinated ligands dictate that there is not room for bidentate coordination of the dithiocarbamate, while in other instances the monodentate coordination mode is isolated en route to further ligand loss and bidentate complexation. 

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