Labile coordination compounds

The Pfeiffer Effect (1) is defined as the change in optical rotation of an optically active system (usually a solution of one enantiomer of an optically active compound, called the "environment substance", dissolved in an optically inactive solvent) upon the addition of a racemic mixture of a dissymmetric, optically labile coordination compound. Much work has been done on this Effect (2 - 8) and several mechanisms have been proposed to explain it, which are described in a review by Schipper (2). It is of interest to note that the Effect can occur with racemic mixtures of certain optically labile complex cations (e.g., D.L-[Zn(o-phen)3]2+) whether the environment substance is anionic (d- -bromo-camphor- -sulfonate), neutral (levo-nicotine), or cationic (d-cinchoninium), The most frequently used solvent for the Pfeiffer Effect is water (10), although the Effect is known to occur in other solvents as well (l.it.6). 

We saw in Section 1.3 how Alfred Werner formulated the modern concept of coordination chemistry, which supramolecular chemistry generalises to a complete coordination chemistry . Prior to Werner s time the chain theory of coordination compounds was popular. The chain and Werner formulations of Co (NIT3) 4CI3 and Co (NH3) 3C13 are shown in Figure 3.1. While both theories predict that Co (NH3) 4C13 will exhibit one labile chloride ion per molecule, the chain theory also predicts that Co(NH3)3C13 will have one labile chloride, while Werner s theory ultimately correctly predicted that the chloride is not labile in this case. 

This essay on the lanthanides has repeatedly drawn attention to a problem of immense importance in both chemistry and biochemistry. The role of water in controlling the stability, the structure, and the lability of coordination compounds. In fact the emphasis extends from coordination compounds to the surfaces of solids47. The role of water is then bound to be extremely important not only in complex chemistry and catalysis but in the growth and properties of crystals and amorphous materials. We can illustrate the problems outside Ln(III) chemistry48,49. ... 

The kinetic lability of ferric siderophores requires that transport experiments be performed with molecules bearing separate radioactive labels in the metal and ligand moieties. As coordination compounds the siderophores are thermodynamically stable and kinetically labile. The formation constants are typically 1030. In the case of ferrichrome the exchange half time at pH 6.3 and 37° is about 10 min (57). Published work (58, 59) with doubly labeled ferric schizokinen in Bacillus mega-terium and ferric aerobactin in A. aerogenes as well as a study of ferric enterobactin in E. coli (60) in each instance suggests a synchronous uptake mechanism for iron and ligand. 

An obvious approach to the preparation of a cyanide-bridged trinuclear species is to employ a mononuclear building block with one labile coordination site and to react it with a cyanometalate building block in a 2 1 ratio (Scheme 1). Many such compounds were reported with Cu(II) ions connected by various types of cyano-metalates, [M(CN)4] (M = Ni(II), Pt(II)), [M(CN)6] [M = Crail),Feamil), Co(lll)], or [M(CN)g] [M = Mo(IV), W(V)] (Table II). 

An important application of the trans effect is the synthesis of specific isomers of coordination compounds. Equations (3) and (4) show how the cis and trans isomers of Pt(NH3)2Cl2 can be prepared selectively by taking advantage of the trans effect order Cl > NH3. This example is also of practical interest because the cis isomer is an important antitumor drug, but the trans isomer is ineffective. In each case the first step of the substitution can give only one isomer. In equation (3) the cis isomer is formed in the second step because the Cl trans to Cl is more labile than the Cl trans to the lower trans effect ligand, ammonia. On the other hand, in equation (4) the first Cl to substitute labilizes the ammonia trans to itself to give the trans dichloride as final product. 

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. 

In contrast to CHEC-II(1996) where only rings which have relatively strong cr-bonds between adjacent atoms were reviewed, syntheses of heterocyclic complexes are also be described in this chapter. The chemistry of such chelates or coordination compounds is very interesting as the carbon-metal bond is labile and subject to various reactions such as insertion, protonation, or substitution. However, even though the synthesis of these intramolecular complexes is described in Section 4.19.9, their physical properties are not reported in this chapter. As the cyclic complex is in equilibrium with its open-chain form, the structural properties of such compounds may not be indicative of the heterocycle ring at all. 

Biirgi, H.-B. Stereochemical lability in crystalline coordination compounds. Trans. Amer. Cryst. Assn. 20, 61-71 (1984). 

Cw-diamminedichloroplatmum(ll), [Pt(NH3)2Cl2], is one of a number of platinum coordination compounds used in the treatment of cancer. Commonly known as cisplatin, this compound has a square planar geometry and labile chloride hgands ... 

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