Color of transition metal ions

Table 16.2 Absorption maxima and colors of transition-metal ions in soda-lime silicate...

Note, however, that this transition is a forbidden one, since it orxurs between levels of the d shell. Therefore the parity does not change. Actually the color of transition metal ions is never intense, as far as this type of transitions, also called crystal-field transitions, are involved. ITie parity selection rule is relaxed by coupling of the electronic transition with vibrations of suitable symmetry [13], In tetrahedral symmetry, however, the centre of symmetry is lacking, and the parity selection rule is also relaxed in another way, viz., by mixing small amounts of opposite-parity... 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

The lanthanides have electrons in partly filled 4/orbitals. Many lanthanides show colors due to electron transitions involving the 4/orbitals. However, there is a considerable difference between the lanthanides and the 3d transition-metal ions. The 4/ electrons in the lanthanides are well shielded beneath an outer electron configuration, (5.v2 5p6 6s2) and are little influenced by the crystal surroundings. Hence the important optical and magnetic properties attributed to the 4/ electrons on any particular lanthanide ion are rather unvarying and do not depend significantly upon the host structure. Moreover, the energy levels are sharper than those of transition-metal ions and the spectra resemble those of free ions. 

Glasses are frequently colored with transition-metal ions such as Mn2+, Ni2+, Co2+, lanthanides, or actinides.2 The addition of Co2+ impurities to silica glass leads to a... 

Introduction 231 Fundamental concepts 233 Electronic structure of transition-metal ions 235 Structural characteristics necessary for complex formation 240 Preparation of metal-complex colorants 248 Isomerism in metal-complex dyes 260 Stability of metal-complex dyes 261 Chromium-related problems in the mordant dyeing of wool 268 References 277... 

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. 

Chapter 6 is devoted to discussing the main optical properties of transition metal ions (3d" outer electronic configuration), trivalent rare earth ions (4f 5s 5p outer electronic configuration), and color centers, based on the concepts introduced in Chapter 5. These are the usual centers in solid state lasers and in various phosphors. In addition, these centers are very interesting from a didactic viewpoint. We introduce the Tanabe-Sugano and Dieke diagrams and their application to the interpretation of the main spectral features of transition metal ion and trivalent rare earth ion spectra, respectively. Color centers are also introduced in this chapter, special attention being devoted to the spectra of the simplest F centers in alkali halides. 

Note that although both color and paramagnetism of transition metal ions are generally associated with the presence of unpaired d electrons,... 

Colored mixed-metal oxide pigments result from the incorporation of color-giving transition metal ions into an oxide host-lattice (see Table 5.9-13). Depending upon the particle size and properties of the chosen host, pigments (0.2 to 2 pm) or ceramic colorants (stains) (up to ca. 10 pm) result, which in many cases are characterized by high thermal and chemical stability and thus are suitable for the coloring of enamels and ceramics. 

Seashells, which are formed in very slow precipitation reactions, are mostly calcium carbonate (CaC03), a white compound. Traces of transition metal ions give them color. 

The petroleum industry uses numerous heterogeneous catalysts. Many of them contain highly colored compounds of transition metal ions. Several are shown here. 

Anhydrous AI2O3 occurs naturally as the extremely hard, high-melting mineral corundum, which has a network structure. It is colorless when pure, but becomes colored when transition metal ions replace a few Al + ions in the crystal. Sapphire is usually blue and contains some iron and titanium. Ruby is red due to the presence of small amounts of chromium. 

The appearance of water, especially textile wastewater is systematically described in terms of its visible characteristics (Standard Methods, 1998). In this regard, the presence of color, suspended particles and turbidity is the focus of much of the testing conducted. In the case of textiles, the presence of color in wastewater is extremely important and numerical methods are normally employed to report results from making color assessments. While color in textile wastewater may arise from the presence of transition metal ions, vegetable matter and industrial plant effluents, color derived from unspent dyebaths is of primary importance. Invariably, this color is removed using a number of physical and/or chemical methods (Reife, 1993) however, methods enabling the recycle/reuse of dye-based color have been developed (Reife and Freeman, 1996). 

The attraction that the polynuclear transition metal cyanides have always had for chemists may be ascribed mainly to the characteristic deep colors of some members of this class of compotmds. These inorganic poisoners offer a rich variation in their stoichiometries and hence in their electronic properties. The broad range of different compounds having the general composition Mt[M (CN) i]j H20 is due to the large number of possible combinations of transition metal ions and M , particularly by considering the different possible oxidation states. As special cases we will find a number of mixed valence compounds, a class of compounds which has attracted widespread attention during the last few years (77, 18). 

Without impurities alumina is colorless. However, addition of transition metal ions to alumina leads to spectacular colors, gem stones, and practical applications such as ruby lasers. Many aspects of color are discussed in detail in [51]. 

The split in energy of transition-metal ion d and / orbitals, as a result of their interaction with their local environment, gives rise to selective absorption in the visible range. It is this absorption that is responsible for the striking colors exhibited by some glasses and gems. 

The color of cement is the result of Ught absorption in the visible part of spectrum, due to the presence of transition metal ions. 

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