Inert coordination compounds

Kinetically inert coordination compounds are discussed in Chapter 12. 

The two best-known therapies involving kinetically inert coordination compounds are chrysotherapy (treatment of rheumatoid arthritis with gold compounds) and the relatively new treatment of certain cancers with platinum compounds. 

To classify the varying rates of reaction (most commonly with regard to substitution) of coordination compounds, Henry Taube, who received the 1983 Nobel Prize in chemistry for his work in the kinetics of coordination compounds, suggested the terms and inert. If we consider a 0.1 Af aqueous solution, a lahk coordination compound is one that under these circumstances has a half-life of less than a minute. (Recall that half-life is the amount of time required for the concentration of the reactant to decrease to half its initial concentration.) An inert coordination compound, on the other hand, is one with a half-life greater than a minute. 

The chemistry of Cr(III) in aqueous solution is coordination chemistry (see Coordination compounds). It is dominated by the formation of kineticaHy inert, octahedral complexes. The bonding can be described by Ss]] hybridization, and HteraHy thousands of complexes have been prepared. The kinetic inertness results from the electronic configuration of the Cr ion (41). This type of orbital charge distribution makes ligand displacement and... 

Pyridine readily forms stable coordination compounds. Thus, boron, aluminum and gallium trihalides react at 0°C in an inert solvent to give 1 1 adducts (cf. 85). Steric factors are important, and a-substituents decrease the ease of reaction. This is illustrated by the heats of reaction of pyridine, 2-methylpyridine and 2,6-dimethylpyridine with boron trifluoride which are 101.3, 94.1 and 73.2 kJ mol-1, respectively. The marked decrease in exothermicity here should be contrasted with the small steric requirement of the proton as shown by the pA., values of substituted pyridines (see Section 3.2.1.3.4). 

Matrix isolation methods of synthesis have also been used to prepare and study coordination compounds. These involve the vaporization of a metal and a potential ligand, which are then rapidly carried in a stream of inert gas to a very cold surface, where the compound which has been formed is quickly trapped in the solid matrix. It is possible to determine the type of bonding, the structure and the thermodynamic properties of the compounds formed. Only small ligand molecules have been used thus far carbon monoxide, nitric oxide, nitrogen and oxygen, for example, but molecules of great interest have been formed. Some such are [Pd(C2H4)], [Pd(N2)3], [Ni(N2)202], [Ni(N2)4] and [Ni(CO)(N2)3].41... 

The coordination chemist may be interested in the electrosynthesis of compounds, the generation and detection of unstable species in unusual oxidation states and the study of their mechanisms of decay or their spectroscopic properties, or in simply obtaining thermodynamic data. These, and related topics such as using electrogenerated metallo intermediates to catalyze the transformation of inert molecules, modifying the properties of an electrode surface by adsorbing or otherwise binding a coordination compound to it, or fundamental aspects of electron-transfer kinetics, are readily studied by the application of modem electrochemical techniques. 

Modern electrochemical methods provide the coordination chemist with a powerful means of studying chemical reactions coupled to electron transfer and exploiting such chemistry in electrosynthesis. In addition, the electrochemical generation of reactive metallo intermediates can provide routes for the activation of otherwise inert molecules, as in the reduction of N2 to ammonia,50 and for electrocatalyzing redox reactions, such as the reduction of C02 to formate and oxalate,51 the oxidation of NH3 to N02-,52 and the technologically important oxidation of water to 02 or its converse, the reduction of 02 to water.53 Electrochemical reactions involving coordination compounds and organometallic species have been extensively reviewed.54-60... 

It is possible to improve the existing techniques of synthesis of PcCu and other metal phthalocyanines from phthalimide or urea and PA, applying electrosynthesis. These processes have many peculiarities. The solvent nature greatly affects the course of the majority of reactions of coordination compound formation [119,120]. The solvent used for PcCu preparation must be inert in order not to influence the desirable reaction course, and simultaneously must have electroconductivity to carry out electrolysis. It is recommended to use the derivatives of urea as such solvents and promoters at the same time. The use of a standard electrochemical procedure [9-14,20,121-124a] could be useful for the PcCu industry, since a typical industrial... 

HPhe preceding and following chapters amply illustrate the reasons why microbial iron transport compounds are worthy of our attention—both from the biochemical and medical points of view. However, one might wonder what this has to do with coordination chemistry. The obvious answer is that these are, after all, coordination compounds. But more than that, when viewed from the perspective of a coordination chemistry, new experiments or new approaches suggest themselves. This is always the exciting potential of interdisciplinary research. This chapter is the result of a research project which has involved extensive collaboration between J. B. Neilands laboratories and my own. Many of the details of the transport studies of kinetically inert, metal-substituted siderophores in... 

Besides the physicochemical studies on solution chemistry, investigations for chemical reactions in solution were extensively developed in the latter half of the 20th century. Studies on chemical reactions in solution were, in principle, closely related to Werner s synthetic works of coordination compounds, and thus, these studies were carried out mostly by coordination chemists rather than physical chemists. Accumulation of knowledge on chemical reactions through studies on synthetic and decomposition reactions led to construction of more reasonable scheme for chemical reactions in solution. Spectrophotometric and optical rotatory dispersive methods, which had been employed in the structural investigations for inert complexes in solution, became important techniques in studies of solution chemistry including reactions of labile complexes. 

The Werner theory of coordination compounds was based on a group of compounds that is relatively slow to react in solution and thus easier to study. For this reason, many of his examples were compounds of Co(III), Rh(III), Cr(III), Pt(II), and Pt(IV), which are kinetically inert or slow to react Examination of more reactive compounds over the years has confirmed their similarity to those originally smdied, so we will include examples of both types of compounds in the descriptions that follow. 

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