The formation of complexes

The coordination number represents the number of spaces available around the central atom or ion in the so-called coordination sphere, each of which can be occupied by one (monodentate) ligand. The arrangement of ligands around the central ion is symmetrical. Thus, a complex with a central atom of a coordination number of 6, comprises the central ion, in the centre of an octahedron, while the six ligands occupy the spaces defined by the vertices of the octahedron. To the coordination number 4 a tetrahedric symmetry normally corresponds, although a planar (or nearly planar) arrangement, where the central ion is in the centre of a square and the four ions occupy the four corners of the latter, is common as well. 

Simple inorganic ions and molecules like NH3, CN , Cl-, H20 form monodentate ligands, that is one ion or molecule occupies one of the spaces available around the central ion in the coordination sphere, but bidentate (like the dipyridyl ion), tridentate, and also tetradentate ligands are known. Complexes made of polydentate ligands are often called chelates, the name originating from the Greek word for the claw of the crab, which bites into an object like the polydentate ligand catches the central ion. The formation of chelate complexes is used extensively in quantitative chemical analysis (complexometric titrations).  

Formulae and names of some complex ions are as follows  

From these examples the rules of nomenclature are apparent. The central atom (like Fe, Cu, Co, Ag) is followed by the formula of the ligand (CN, NH3, H20, S203), with the stoichiometric index number, (which, in the case of monodentate ligands is equal to the coordination number). The formula is placed inside square brackets, and the charge of the ion is shown outside the brackets in the usual way. When expressing concentrations of complexes, brackets of the type will be used to avoid confusion. In the name of the ion, first the (Greek) number, then the name of the ligand is expressed, followed by the name of the central atom and its oxidation number (valency). 

The classical rules of valency do not apply for complex ions. To explain the particularities of chemical bonding in complex ions, various theories have been developed. As early as 1893, A. Werner suggested that, apart from normal valencies, elements possess secondary valencies which are used when complex ions are formed. He attributed directions to these secondary valencies, and thereby could explain the existence of stereoisomers, which were prepared in great numbers at that time. Later G. N. Lewis (1916), when describing his theory of chemical bonds based on the formation of electron pairs, explained the formation of complexes by the donation of a whole electron pair by an atom of the ligand to the central atom. This so-called dative bond is sometimes denoted by an arrow, showing the direction of donation of electrons. In the structural formula of the tetramminecuprate(II) ion 

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The ability to form hydrogen bonds explains the formation of complex ions such as HFJ and HjFj when a fluoride salt, for example potassium fluoride, is dissolved in aqueous hydrofluoric acid ... 

Separation Processes. The product of ore digestion contains the rare earths in the same ratio as that in which they were originally present in the ore, with few exceptions, because of the similarity in chemical properties. The various processes for separating individual rare earth from naturally occurring rare-earth mixtures essentially utilize small differences in acidity resulting from the decrease in ionic radius from lanthanum to lutetium. The acidity differences influence the solubiUties of salts, the hydrolysis of cations, and the formation of complex species so as to allow separation by fractional crystallization, fractional precipitation, ion exchange, and solvent extraction. In addition, the existence of tetravalent and divalent species for cerium and europium, respectively, is useful because the chemical behavior of these ions is markedly different from that of the trivalent species. 

Silver Chloride. Silver chloride, AgCl, is a white precipitate that forms when chloride ion is added to a silver nitrate solution. The order of solubility of the three silver halides is Cl" > Br" > I. Because of the formation of complexes, silver chloride is soluble in solutions containing excess chloride and in solutions of cyanide, thiosulfate, and ammonia. Silver chloride is insoluble in nitric and dilute sulfuric acid. Treatment with concentrated sulfuric acid gives silver sulfate. 

Thiosulfates. The ammonium, alkaU metal, and aLkaline-earth thiosulfates are soluble in water. Neutral or slightly alkaline solutions containing excess base or the corresponding sulfite are more stable than acid solutions. Thiosulfate solutions of other metal ions can be prepared, but their stabiUty depends on the presence of excess thiosulfate, the formation of complexes, and the prevention of insoluble sulfide precipitates. 

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

A.uxilia driers do not show catalytic activity themselves, but appear to enhance the activity of the active drier metals. It has been suggested that the auxihary metals improve the solubiUty of the active drier metal, can alter the redox potential of the metal, or function through the formation of complexes with the primary drier. Auxihary driers include barium, zirconium, calcium, bismuth, zinc, potassium, strontium, andhthium. 

The other halides of Zn and Cd are in general hygroscopic and very soluble in water ( 400g per lOOcm for ZnXa and 100g per lOOcm for CdXa). This is at least partly because of the formation of complex ions in solution, and the anhydous forms are best prepared by... 

Interesting tautomeric possibilities exist in the xanthobilirubic acid series (cf. reference 57) which can be illustrated by the equilibrium 62 63, More complex examples of the same type are found among the linear tetrapyrrole pigments— the bilenes, bilidienes, and bili-trienes—and have been discussed by Stevens. Relatively little evidence is available concerning the fine structure of these compounds, although the formation of complexes has been advanced as evidence for the 0X0 structure in some cases. ... 

The formation of complex mixtures of products by a Prins reaction can be a problem. An example is the reaction of aqueous formaldehyde with cyclohexene 8 under acid catalysis ... 

In the pulp and paper industry, anionic and cationic acrylamide polymers are used as chemical additives or processing aids. The positive effect is achieved due to a fuller retention of the filler (basically kaoline) in the paper pulp, so that the structure of the paper sheet surface layer improves. Copolymers of acrylamide with vi-nylamine not only attach better qualities to the surface layer of.paper, they also add to the tensile properties of paper in the wet state. Paper reinforcement with anionic polymers is due to the formation of complexes between the polymer additive and ions of Cr and Cu incorporated in the paper pulp. The direct effect of acrylamide polymers on strength increases and improved surface properties of paper sheets is accompanied by a fuller extraction of metallic ions (iron and cobalt, in addition to those mentioned above), which improves effluent water quality. 

The stability of tin over the middle pH range (approximately 3-5-9), its solubility in acids or alkalis (modified by the high hydrogen overpotential), and the formation of complex ions are the basis of its general corrosion behaviour. Other properties which have influenced the selection of tin for particular purposes are the non-toxicity of tin salts and the absence of catalytic promotion of oxidation processes that may cause changes in oils or other neutral media affecting their quality or producing corrosive acids. 

On the other hand, the presence of CN ions greatly increases the zone of corrosion, owing to the formation of complex ions. Silver, therefore, is thermodynamically stable in reducing acids, e.g. hydrochloric acid, acetic acid, phosphoric acid, provided oxidising substances are absent. 

All the six platinum-group metals are highly resistant to corrosion by most acids, alkalis, and other chemicals. Their high nobility is the main factor determining their chemical resistance, and the formation of. complex ions in solution is principally responsible for their dissolution under certain conditions. 

As with the chemical behaviour of the noble metals in aqueous solutions, their anodic behaviour closely follows the predictions of the Pourbaix diagrams if due allowance is made for the formation of complexes. 

As feed systems usually contain copper alloys, the use of amines for their protection may seem somewhat strange as copper is prone to attack in ammonia/carbon dioxide/oxygen environments, with the formation of complex cupric or cuprous compounds. The requisite degree of protection can be achieved, however, by maintaining the concentrations strictly within the acceptable target range. 

In most cases, the formation of complexes in molten salts leads to an increase in the molar volume relative to the additive volume. This phenomenon is usually explained by an increase in bond covalency. Nevertheless, the nature of the initial components should be taken into account when analyzing deviations in property values, as was shown by Markov, Prisyagny and Volkov [314]. In particular, this rule applies absolutely when the system consists of pure ionic components. The presence of initial components with a significant share of covalent bonds leads to an S-shaped isotherm [314]. 

Electro-conductivity of molten salts is a kinetic property that depends on the nature of the mobile ions and ionic interactions. The interaction that leads to the formation of complex ions has a varying influence on the electroconductivity of the melts, depending on the nature of the initial components. When the initial components are purely ionic, forming of complexes leads to a decrease in conductivity, whereas associated initial compounds result in an increase in conductivity compared to the behavior of an ideal system. Since electro-conductivity is never an additive property, the calculation of the conductivity for an ideal system is performed using the well-known equation proposed by Markov and Shumina (Markov s Equation) [315]. 

Based on Equation (55), it was concluded/assumed that niobium-containing oxyfluoride melts are characterized by the formation of complex ions NbOF63. ... 

The formation of complexes in fluoride and oxyfluoride melts containing tantalum and niobium will be discussed later on in detail. 

Volkov, Grischenko and Delimarsky [293] mentioned a similar tendency of the complexes to increase in strength with the increase in the polarity of ligands along the sequence F to I. This concept confirms the significance of the covalent share in the energetics of the formation of complex ions in molten media. 

Fe+ + has been deprotonated, but the reaction is complicated by further nucleophilic attack of the methylene unit with the starting material [17]. Enhanced acidity of the ring hydrogens in arene-metal complexes is shown [21] by the formation of complexes of alkyllithium by proton abstraction. 

The double substitution of both chlorine atoms in the complex of o-dichlor-obenzene can, under certain conditions, lead to the formation of complexes of heterocycles [99, 100, 104] Scheme XX ... 

The hydrolysis of 106 was studied in some detail and led to the isolation and structural characterization of a rare example of a titanoxane derivative with a planar Ti2(p-G)2 core 108 [111]. The formation of complex 108 can be contrasted with the hydrolysis reaction of the trihalides in the Cp and Cp series giving rise... 

Abnormally low atomic heats were explained by Richarz on the assumption of a diminution of freedom of oscillation consequent on a closer approximation of the atoms, which may even give rise to the formation of complexes. This is in agreement with the small atomic volume of such elements, and with the increase of atomic heat with rise of temperature to a limiting value 6 4, and the effect of propinquity is seen in the fact that the molecular heat of a solid compound is usually slightly less than the sum of the atomic heats of the elements, and the increase of specific heat with the specific volume when an element exists in different allotropic forms. 

The formation of complex silicate scales takes place at high temperature (usually they are only found in boilers operating at over 300 psig) and high heat-flux density points in the boiler section. The presence of complex silicates such as analcite and acmite may indicate steam blanketing problems. 

Unfortunately, for ligands of strong acids, this equation may underestimate the stability constant as it calculates values for inner sphere formation only. Eigen (22) has proposed that the formation of complexes proceeds sequentially as follows ... 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

It is interesting to note, as pointed out to me by Mr. J. L. Hoard, that these considerations lead to an explanation of the stability of trivalent cobalt in electron-pair bond complexes as compared to ionic compounds. The formation of complexes does not change the equilibrium between bivalent and trivalent iron very much, as is seen from the electrode potentials, while a great change is produced in the equilibrium between bivalent and trivalent cobalt. 

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