Ion-complexes

Chromium ions are sensitive to their Chemical environment. 

When a tetrahedron is reflected or rotated, a structure indistinguishable from the original is obtained. Chemists call the tetrahedron a symmetric structure. Escher, in his painting Symmetry No. 20, shows a translational repeating symmetry. He flips or moves his objects, creating a pattern. 

It has been known for more than a century, however, that transition metals also form a variety of ionic compounds with more complex formulas such as 

In these so-called coordination compounds, the transition metal is present as a complex ion, enclosed within the brackets. In the three compounds listed above, the following complex ions are present  

The charges of these complex ions are balanced by those of simple anions or cations (e.g., S042-, 3CI-, 3K+). 

Complexes of transition metal ions with a formally empty d shell often show intense broadband emission with a large Stokes shift of 10,000-20,000 cm k The most important examples for minerals are VO , WO , MoO and TiOg . Atomic orbitals s,p, d of the central atom andp orbitals of oxygen form molecular orbitals of the complexes (Fig. 5.60). The excited state is considered to 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is 

Pyromorphite usually has an orange to yellow UV excited liuninescence characterized by a broadband peaking at 580 nm (Fig. 4.26). It was earlier supposed 

The blue emission band in the beryl liuninescence spectrum with a short decay time of 1 ps (Fig. 4.52a) can be connected with the (V04) complex, especially because a similar emission has been foimd in the radioluminescence spectrum of beryl with an elevated concentration of vanadium (Chithambo et al. 1995). 

Uririe 3.1. Decay times r of. some luminescent compounds with / metal ions at 4.2 K. 

Although it was believed for years that the emission. spectra of these species were fully structureless, in recent years vibrational structure has been reported for several cases. A beautiful example is given in Fig. 3.19. This relates to vanadate on a silica surface. The progression is in a vibrational mode with a frequency of 950 cm . This is the stretching vibration between vanadium and the oxygen pointing out of the silica surface. 

The emission transitions of ions with configuration are of a complicated nature and are only partly understood. For clarity these ions are here divided inio two classes. 

The Stokes shift of the Cu emission is usually large ( 5000 cm ), indicating the strong-coupling scheme. About the Ag ion less is known, but what is known shows a similarity with the Cu data. 

Complexes of transition metal ions with a formally empty d shell often show intense broadband emission with a large Stokes shift of 10,000-20,000 cm 

Cations are Lewis acids, and anions are Lewis bases, and their respective bonding strengths are also their Lewis acid and Lewis base strengths. So far this discussion has focused on simple ions, that is, ions that consist of a single atom, in which the Lewis acid and base strengths can be calculated using Eq. (7). Complex ions are those composed of more than one atom. They differ from simple ions in that the different atoms in the complex may each have their own Lewis acid or base strength. 

Each atom in a complex ion is necessarily bonded to one or more atoms within the complex to form a network of strong internal (covalent) bonds. For example, the sulfur in the sulfate anion, 864 , is a hexavalent cation, linked by bonds 

Neutral molecules, such as water or ammonia, if they have any chemical activity, must contain both Lewis acid and Lewis base functions, with the condition that their residual positive valence must be numerically equal to their residual negative valence. A consequence is that the Lewis acid and base functions must work together in neutral molecules the total valence of the external bonds formed by the Lewis acid functions must equal the total valence of the external bonds formed by the Lewis base functions. It is helpful to distinguish the Lewis acid and Lewis base functions by arrows on the bonds directed from the acid to the base. 

Complex ions and molecules have more flexibility in adapting to their environment than simple ions since they can redistribute their residual valence among the different atoms that form the surface of the complex ion by changing the valences of the internal bonds. Both the bonding strengths displayed by such complexes, and their internal geometries, may vary depending on what counterions are present in the compound. 

The activity coefficient ya of the undissociated acid is approximately unity in dilute aqueous solution. Expression (24) thus becomes  

If a buffer solution is diluted, the ionic concentrations are decreased and so, as shown in Section 2.5, the ionic activity coefficients are increased. It follows from equation (26) that the pH is increased. 

Buffer mixtures are not confined to mixtures of monoprotic acids or monoacid bases and their salts. We may employ a mixture of salts of a polyprotic acid, e.g. NaH2P04 and Na2HP04. The salt NaH2P04 is completely dissociated  

Buffer solutions find many applications in quantitative analysis, e.g. many precipitations are quantitative only under carefully controlled conditions of pH, as are also many compleximetric titrations numerous examples of their use will be found throughout the book. 

The increase in solubility of a precipitate upon the addition of excess of the precipitating agent is frequently due to the formation of a complex ion. A 

Submitted by WILLIAM E. HATFIELD, ROBIN WHYMAN, ROBERT C. FAY,t KENNETH N. RAYMOND,J and FRED BASOLOt 

Checked by GEORGES. KAUFFMAN, SHAN YAW EEE, and LILY HU CHOWS 

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Also an atom, molecule, or ion that is electron deficient and which can form a co-ordinate link with an electron donor. Thus in the complex ion [Co(NH3)eP the cobalt(Ill) ion is an acceptor and the ammonia the electron donor. t-acceptors are molecules or atoms which accept electrons into n, p or d orbitals. 

Antimony trifluoride, Sbp3, m.p. 292°C, sublimes319 C(Sb203 in HFsolution). Forms complex ions, e.g. [SbFJ". Widely used as a fairly mild fluorinating agent. 

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

More complex ions are created lower in the atmosphere. Almost all ions below 70-80 km are cluster ions. Below this altitude range free electrons disappear and negative ions fonn. Tln-ee-body reactions become important. Even though the complexity of the ions increases, the detemiination of the final species follows a rather simple scheme. For positive ions, fomiation of H (H20) is rapid, occurring in times of the order of milliseconds or shorter in the stratosphere and troposphere. After fomiation of H (H20), the chemistry involves reaction with species that have a higher proton affinity than that of H2O. The resulting species can be... 

The aluminium ion, charge -I- 3. ionic radius 0.045 nm, found in aluminium trifluoride, undergoes a similar reaction when a soluble aluminium salt is placed in water at room temperature. Initially the aluminium ion is surrounded by six water molecules and the complex ion has the predicted octahedral symmetry (see Table 2.5 ) ... 

This complex ion behaves as an acid in water, losing protons, and a series of equilibria are established (H is used, rather than for simplicity) ... 

Central unit Ligand Co-ordination number Ligand type Complex ion Shape... 

When naming complex ions the number and type of ligands is written first, followed by the name of the central metal ion. If the complex as a whole has a positive charge, i.e. a cation, the name of the central metal is written unchanged and followed by the oxidation state of the metal in brackets, for example [Cu(N 113)4] becomes tetra-ammine copper(II). A similar procedure is followed for anions but the suffix -ate is added to the central metal ion some examples are ... 

It is believed that an intermediate complex ion [AICI4] is formed thus ... 

Silicon, germanium, tin and lead can make use of unfilled d orbitals to expand their covalency beyond four and each of these elements is able (but only with a few ligands) to increase its covalency to six. Hence silicon in oxidation state -f-4 forms the octahedral hexafluorosilicate complex ion [SiFg] (but not [SiCl] ). Tin and lead in oxidation state -1-4 form the hexahydroxo complex ions, hexahydroxostannate(IV). [Sn(OH) ] and hexahydroxoplum-bate(IV) respectively when excess alkali is added to an aqueous solution containing hydrated tin(IV) and lead(IV) ions. 

The small fluoride ion shows a great tendency to act as a ligand and form complex ions, for example [AIF ] , [PF ], [FeFg] in... 

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 ... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

Halogens can act as ligands and are commonly found in complex ions the ability of fluorine to form stable complex ions with elements in high oxidation states has already been discussed (p. 316). However, the chlorides of silver, lead(Il) and mercury(l) are worthy of note. These chlorides are insoluble in water and used as a test for the metal, but all dissolve in concentrated hydrochloric acid when the complex chlorides are produced, i.e. [AgCl2] , [PbC ] and [Hg Clj]", in the latter case the mercury(I) chloride having also disproportionated. 

The d orbital splitting depends on the oxidation state of a given ion hence twb complex ions with the same shape, ligands and coordination number can differ in colour, for example... 

Although the complex ion [MnClg] is unstable, salts such as K2[MnF6] (containing the octahedral hexafluoromanganate(IV) ion) are much more stable and can be crystallised from solution. 

The film is now fixed by washing in sodium thiosulphate ( hypo ) solution when the unchanged bromide is dissolved to form the complex ion... 

In empirical formulas of inorganic compounds, electropositive elements are listed first [3]. The stoichiometry of the element symbols is indicated at the lower right-hand side by index numbers. If necessary, the charges of ions are placed at the top right-hand side next to the element symbol (e.g., S "). In ions of complexes, the central atom is specified before the ligands are listed in alphabetical order, the complex ion is set in square brackets (e.g., Na2[Sn(OH)+]). 

Corey E J and J C Bailar ]t 1959. The Stereochemistry of Complex Inorganic Compounds. XXII. Stereospecific Effects in Complex Ions. Journal of the American Chemical Society 81 2620-2629. 

Free Ions Versus Complexed Ions In discussing the ion-selective electrode, we noted that the membrane potential is influenced by the concentration of F , but not the concentration of HF. An analysis for fluoride, therefore, is pH-dependent. Below a pH of approximately 4, fluoride is present predominantly as HF, and a quantitative analysis for total fluoride is impossible. If the pH is increased to greater than 4, however, the equilibrium... 

In Section 4.2.1 it will be pointed out that hydrogen peroxide (Figure 4.1 la) has only one symmetry element, a C2 axis, and is therefore a chiral molecule although the enantiomers have never been separated. The complex ion [Co(ethylenediamine)3], discussed in Section 4.2.4 and shown in Figure 4.11(f), is also chiral, having only a C3 axis and three C2 axes. 

Ion-exchange separations can also be made by the use of a polymer with exchangeable anions in this case, the lanthanide or actinide elements must be initially present as complex ions (11,12). The anion-exchange resins Dowex-1 (a copolymer of styrene and divinylben2ene with quaternary ammonium groups) and Amherlite IRA-400 (a quaternary ammonium polystyrene) have been used successfully. The order of elution is often the reverse of that from cationic-exchange resins. 

Special techniques for experimentation with the actinide elements other than Th and U have been devised because of the potential health ha2ard to the experimenter and the small amounts available (15). In addition, iavestigations are frequently carried out with the substance present ia very low coaceatratioa as a radioactive tracer. Such procedures coatiaue to be used to some exteat with the heaviest actinide elements, where only a few score atoms may be available they were used ia the earHest work for all the transuranium elements. Tracer studies offer a method for obtaining knowledge of oxidation states, formation of complex ions, and the solubiHty of various compounds. These techniques are not appHcable to crystallography, metallurgy, and spectroscopic studies. 

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