Complex ions colors

From the color (absorption spectrum) of a complex ion, it is sometimes possible to deduce the value of AOJ the crystal field splitting energy. The situation is particularly simple in 22Ti3+, which contains only one 3d electron. Consider, for example, the Ti(H20)63+ ion, which has an intense purple color. This ion absorbs at 510 nm, in the green region. The... 

When this reaction is complete, the slightest excess of thiocyanate produces a reddish-brown coloration, due to the formation of a complex ion ... 

In aqueous solution, water competes effectively with bromide ions for coordination to Cir+ ions. The hexaaquacopper(II) ion is the predominant species in solution. However, in the presence of a large concentration of bromide ions, the solution becomes deep violet. This violet color is due to the presence of the tetrabromocuprate(Il) ions, which are tetrahedral. This process is reversible, and so the solution becomes light blue again on dilution with water, (a) Write the formulas of the two complex ions of copper(II) that form, (b) Is the change in color from violet to blue on dilution expected Explain your reasoning. 

A CT transition which is very similar to the -> MMCT transition has been observed by Vogler et al. [55] for complexes [M(2,2 -bipyridyl)X3] with X = Cl, Br, I and M = Sb, Bi. These authors report MLCT transitions involving the promotion of an electron from the lone pair to the n orbital of the bipyridyl ligand. For example, for M = Sb and X = Br they observe an orange color for the complex due to an absorption band with a maximum at 435 nm. In the complexes considered by us the transition is to an antibonding n orbital (with pronounced d character) on the filled-shell transition-metal complex ion. 

It may be pointed out that salt solutions owe their color to the ions which they yield when dissolved in water. Thus, Cu2+, CrO2-, MnO2- and Co2+ ions in the bulk are blue, yellow, purple, and pink, respectively, and any solution of a salt containing one of these ions will have the same absorption spectrum. If the colored ion is removed from solution by the formation of a colorless complex ion, the solution will lose its color, e.g., by addition of KCN to CuS04. 

Acid-base (neutralization) reactions are only one type of many that are applicable to titrimetric analysis. There are reactions that involve the formation of a precipitate. There are reactions that involve the transfer of electrons. There are reactions, among still others, that involve the formation of a complex ion. This latter type typically involves transition metals and is often used for the qualitative and quantitative colorimetric analysis (Chapters 8 and 9) of transition metal ions, since the complex ion that forms can be analyzed according to the depth of a color that it imparts to a solution. In this section, however, we are concerned with a titrimetric analysis method in which a complex ion-forming reaction is used. 

A good example of a bidentate ligand is the 1,10-phenanthroline molecule, which, since it forms a stable complex ion with Fe2+ ions that is deep orange color, is used in the colorimetric analysis of iron(II) ions (see Experiment 19 in Chapter 7). This ligand is shown in Figure 5.17(a). The two bonding sites are... 

At Los Alamos National Laboratory, the silicon content of plutonium samples is determined spectrophotometrically using a silica-molybdenum blue complex ion, which has a blue color. The sample is dissolved in a mixture of hydrochloric acid and hydrofluoric acid. After the excess hydrofluoric acid is removed, the absorbance is measured at 825 nm. All operations are carried out in a silica-free apparatus. 

These reagents are required for proper color development. Hydroxylamine hydrochloride is a reducing agent, which is required to keep the iron in the +2 state. The o-phenanthroline is a ligand that reacts with Fe2+ to form an orange-colored complex ion. This ion is the absorbing species. In addition, since the reaction is pH dependent, sodium acetate is needed for buffering at the optimum pH. 

The three elements to be treated in this chapter (V, Cr, Mn) are the third, fourth, and hfth members of the first transition series. The first two members (Sc, Ti) have been treated in previous chapters (Chapters 12 and 13). The ten elements of this first transition series (Sc through Zn) are characterized by electron activity in the 3d-4s levels. All elements in the 3d transition series are metals, and many of their compounds tend to be colored as a result of unpaired electrons. Most of the elements have a strong tendency to form complex ions due to participation of the d electrons in bonding. Since both the 4s and the 3d electrons are active, most of the elements show a considerable variety of oxidation states (Sc and Zn being exceptions). For the first five (Sc through Mn), the maximum oxidation number is the total number of electrons in the 4s and 3d levels. Complexing is often so strong that the most stable oxidation state for simple compounds may differ from that for complex compounds. 

The reaction appears to be applicable to a wide range of aromatic starting materials, exceptions being aromatic aldehydes and aromatic primary amines [96]. In effect, the aromatic compound to be converted is stirred with an aqueous solution of hydroxylamine hydrochloride in the presence of metallic ions such as copper ions [95-97] or complex ions such as the pentacyanoam-mine ferrate(II) ion [98]. The mixture is then treated with 30% hydrogen peroxide and a highly colored complex of the nitrosophenol forms. Presumably, the free nitrosophenol may be isolated by treatment of the complex with an acid (Eq. 50). 

The characteristic light purple color of [Cr(OH2)6]3+ in chrom alum, K2S04 Cr2(S04)3 12H20, absorption maxima 575 (13.2), 408 (15.5), I = 1.0 M (70), is frequently masked by complex ion formation in other salts, e.g., the green form of CrCl3 6H20, which contains predominantly [Pg.357]

Addition of aqueous HgCl2 also eliminates the red color because Hg2+ reacts with SCN ions to form the stable Hg-S bonded complex ion Hg( SCN)42-. Removal of free SCN (aq) shifts the equilibrium Ve3+(aq) + SCN (aq) FeNCS2+(ag) from right to left to replenish the SCN ions. 

Lee R. Summerlin, Christie L. tBorgford, and Julie B. Ealy, "Colorful Complex Ions in Ammonia," Chemical Demonstrations, A Sourcebook for Teachers, Vol. 2 (American Chemical Society, Washington, DC, 1988), pp. 75-76. 

R. A. Pacer, "Will a precipitate form Will it dissolve " J. Chem. Educ., Vol. 71,1994,69. Principles of ionic compound precipitate formation are shown in a series of four colorful demonstrations (1) Yellow Pbl2 is formed only when Q > Ksp. (2) Upon addition of aqueous ammonia, green NiC03(s) is dissolved due to the formation of the blue complex ion [Ni(NH3)6]2+. 

The silver chloride (AgCl) is dissolved as the stable silver-ammonia-complex ion [Ag(NH3) 2+] is formed. Meanwhile, the mercury II chloride (HgCl2) is undergoing oxidation and reduction at the same time Mercury metal (Hg) and mercury II amidochloride (HgNH2Cl) are formed and appear, respectively, black and gray in color ... 

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