Coordination complexes an introduction

Our discussions in Section 6.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

At room temperature, the solubilities in water of LiI03, NaI03 and KIO3 are 4.4, 0.45 and 0.22moldm, respectively. Rationalize this trend. 

So far, we have focused on aqueous solutions containing a single, dissolved ionic salt, MX. Now we consider the effect of adding a second salt which has one of its ions in common with the first salt. 

If a salt MX is added to an aqueous solution containing the solute MY (the ion M is common to both salts), the presence of the dissolved M ions suppresses the dissolution of MX compared with that in pure water this is the common-ion effect. 

Now consider the solubility of AgCl in 0.0100 mol dm aqueous HCl solution. HCl is essentially fully dissociated and thus, [Cl ] = 0.0100 mol dm .  

In this section we introduce some general principles concerning the coordination of ligands to ions in aqueous solution. These definitions and principles will be used again when we discuss complex formation in detail later in the book. The 

In a coordination complex, a central atom or ion is coordinated by one or more molecules or ions ligands) which act as Lewis bases, forming coordinate bonds with the central atom or ion the latter acts as a Lewis acid. Atoms in the ligands that are directly bonded to the central atom or ion are donor atoms. 

Examples of coordination complexes include those involving rf-block metal ions (e.g. [Co(NH3)6], 6.12) and species with a central j-block element (e.g. [BF4] , 6.13, and H3B-THF, 6.14) (THF = tetrahydrofuran) although 6.14 is unstable with respect to hydrolysis in aqueous solution. Equations 6.59-6.61 show the formation of these coordination complexes. 

When a Lewis base donates a pair of electrons to a Lewis acid, a coordinate bond is formed and the resulting species is an adduct. The centred dot in, for example, H3B THF indicates the formation of an adduct. 

In [BF4], the B—F bond formed in reaction 6.60 is identical to the other three B—F bonds aU are 2c-2e covalent bonds. In structures 6.12-6.14, the coordinate bond between the central atom or ion and a neutral ligand is denoted by 

In [Bp4] , the B—F bond formed in reaction 7.60 is identical to the other three B—F bonds all are 2c-2e covalent bonds. In structures 7.12-7.14, the coordinate bond between the central atom or ion and a neutral ligand is denoted by an arrow, but if the ligand is anionic, the coordinate bond is indicated by a hne. This convention is sometimes ignored, for example, when the stereochemistry of the coordination complex is illustrated compare 7.12 with 7.15 which shows the octahedral environment of the Co(II) centre. 

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Notice the loose use of the term octahedral to describe six-coordinate complexes which are based upon an octahedral geometry, but which, by virtue of the presence of different ligand types, are of lower symmetry than Oh. This is a common usage which should give rise to no difficulties. Note also how introduction of chelating... 

As an introduction to the theory as it relates to these defect complexes, we point out that the most conspicuous experimental feature of a light impurity such as hydrogen is its high local-mode frequency (Cardona, 1983). Therefore, it is essential that the computational scheme produce total energies with respect to atomic coordinates and, in particular, vibrational frequencies, so that contact with experiment can be established. With total-energy capabilities, equilibrium geometries and migration and reorientation barriers can be predicted as well. 

Although changes in pM can readily be followed by physical means (e.g. potentiometrically), following colour changes associated with the formation and dissociation of metal coordination complexes visually or spectrophotometrically is a more versatile and convenient procedure. The serendipitous discovery of so-called metallochromic indicators made by G. Schwarzenbach (1945) led immediately to the introduction of murexide (50) as an indicator in calcium titrations and initiated the search for indicators for other metal-EDTA systems. It will be realized that the chosen metal indicator must be considerably less stable than the metal-EDTA complex, but not so weak as to dissociate appreciably in the vicinity of the end-point when the concentration of free metal... 

The material presented in this chapter provides an introduction to the vast area of reactions of coordination compounds. In addition to the types of reactions described, there is an extensive chemistry of reactions of coordinated ligands. Because many ligands are organic molecules, it is possible to carry out reactions on the ligands without disruption of the complexes, and some such reactions have been mentioned in this chapter. Several others will be shown in Chapter 21. Many reactions of coordination compounds have been studied in detail, and much is known about processes taking place in both solids and solutions. However, it is not possible in a book such as this to do more than introduce the field, but the listed references provide a basis for further study of this area. 

Another characteristic of modem coordination chemistry is the increasing reliance upon physicochemical methods unknown to Werner and his contemporaries. Simultaneously with an introduction of these newer techniques, emphasis shifted from preoccupation with qualitative studies of stmcture and stereochemistry to quantitative studies of thermodynamics, kinetics, and reaction mechanisms. Some areas of current research interest include unusual ligands, oxidation states and coordination numbers, solid-state chemistry, photochemistry, relationship between stmcture and reactivity, variable oxidation state, chelates, heteropoly complexes, organometalhc... 

Schwarz, Robert, Lawrence W. Bass, and Alfred Werner. The Chemistry of the Inorganic Complex Compounds An Introduction to Werner s Coordination Theory. New York Wiley, 1923. 

Stability constants for metal complexes (Section 6.12) selected ligand structures and abbreviations (Table 6.7) an introduction to coordination complexes (Section 6.11) ... 

For an introduction to the use of [Ni(Hdmg)2] and related complexes in template syntheses of macrocycles, see E.C. Constable (1999) Coordination Chemistry of Macrocyclic Compounds, Oxford University Press, Oxford (Chapter 4). 

For a detailed review, see A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. von Zelewsky (1988) Coordination Chemistry Reviews, vol. 84, p. 85 - Ru(II) polypyridine complexes Photophysics, photochemistry, electrochemistry and chemiluminescence . For an introduction to photochemical principles, see C.E. Wayne and R.P. Wayne (1996) Photochemistry, Oxford University Press Primer Series, Oxford. 

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