Transition elements chromium

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

While the energy level diagrams and the corresponding spectroscopic transitions are relatively straightforward for light elements, they become very complex for heavier elements. The number of observed Hnes listed by Harvey [3] increases from the alkah metals with 30 (for lithium) to 645 (for caesium) to several thousand lines for the transition elements (chromium 2277, iron 4757, and cerium 5755 lines, respectively). 

In this section, we look at the chemical properties of two transition elements, chromium and copper. Both elements are well known and commercially important in addition, they provide many examples of colorful compounds. 

Polyatomic Anions from Transition Elements Chromium and manganese form some polyatomic anions that theoretically may be traced to acids, but only the ions are important. Their names and formulas are the chromate ion, CrO/ , the dichromate ion, Cr207 , and the permanganate ion, Mn04. ... 

Selected Top-Row Transition Elements Chromium, Manganese, Iron, Cobalt,... 

By reference to the outline periodic table shown on p. (i) we see that the metals and non-metals occupy fairly distinct regions of the table. The metals can be further sub-divided into (a) soft metals, which are easily deformed and commonly used in moulding, for example, aluminium, lead, mercury, (b) the engineering metals, for example iron, manganese and chromium, many of which are transition elements, and (c) the light metals which have low densities and are found in Groups lA and IIA. 

Possibly because of price and performance competition from chromium, titanium, and other transition elements, only about a dozen vanadium compounds are commercially significant of these, vanadium pentoxide is dominant. 

Finally, in 1797, the Frenchman L. N. Vauquelin discovered the oxide of a new element in a Siberian mineral, now known as crocoite (PbCr04), and in the following year isolated the metal itself by charcoal reduction. This was subsequently named chromium (Greek xpco ia, chroma, colour) because of the variety of colours found in its compounds. Since their discoveries the metals and their compounds have become vitally important in many industries and, as one of the biologically active transition elements, molybdenum has been the subject of a great deal of attention in recent years, especially in the field of nitrogen fixation (p. 1035). 

As you can see from Figure 6.9, the electron configurations of several elements (marked ) differ slightly from those predicted. In every case, the difference involves a shift of one or, at the most, two electrons from one sublevel to another of very similar energy. For example, in the first transition series, two elements, chromium and copper, have an extra electron in the 3d as compared with the 4s orbital. 

The d block includes all the transition elements. In general, atoms of d block elements have filled ns orbitals, as well as filled or partially filled d orbitals. Generally, the ns orbitals fill before the (n - l)d orbitals. However, there are exceptions (such as chromium and copper) because these two sublevels are very close in energy, especially at higher values of n. Because the five d orbitals can hold a maximum of ten electrons, the d block spans ten groups. 

Many of the compounds formed by transition elements appear in various colors. Several are very toxic. Chromium, zinc, cobalt, nickel, and titanium are carcinogenic. 

Chromium is a silvery white/gray, hard, brittle noncorrosive metal that has chemical and physical properties similar to the two preceding elements in period 4 (V andTi). As one of the transition elements, its uses its M shell rather than its outer N shell for valence electrons when combining with other elements. Its melting point is 1,857°C, its boiling point is 2,672°C, and its density is 7.19 g/cm. ... 

The classical cases of distinction between valency and oxidation state occur in the coordination complexes 0f the transition elements. For example, in the complex compound [Cr(NH3)63+](Cl )3 the complex ion containing the chromium ion, [Cr(NH3)6]3 +, has a chromium atom at its centre which... 

From various sources Dowden (27) has accumulated data referring to the density of electron levels in the transition metals and finds an increase from chromium to iron. The density is approximately the same from a-iron to /3-cobalt there is a sharp rise between the solid solution iron-nickel (15 85) and nickel, and a rapid fall between nickel-copper (40 60) and nickel-copper (20 80). From Equation (2), the rates of reaction can be expected to follow these trends of electron densities if positive ion formation controls the rates. On the other hand, both trends will be inversely related if the rates are controlled by negative ion formation. Where the rate is controlled by covalent bond formation, singly occupied atomic orbitals are deemed necessary at the surface to form strong bonds. In the transition metals where atomic orbitals are available, the activity dependence will be similar to that given for positive ion formation. In copper-rich alloys of the transition elements the activity will be greatly reduced, since there are no unpaired atomic d-orbitals, and for covalent bond formation only a fraction of the metallic bonding orbitals are available. 

Compared with other transition elements,1092 few complexes of Schiff s bases or /5-keto amines with chromium(III) are known. Most work has been done with ligands (228) to (231) and their complexes and the interrelations between the more important of them are set out in Table 94 and Schemes 103 and 104. 

Molybdenum and tungsten are similar chemically, although there are differences which it is difficult to explain. There is much less similarity in comparisons with chromium. In addition to the variety of oxidation states there is a wide range of stereochemistries, and the chemistry is amongst the most complex of the transition elements. 

Mercury-transition metal bonds have been described for all members of Groups V-VIII of the transition series except, apparently, technetium. They commonly involve a low oxidation state of the transition element and are particularly numerous for the chromium, iron and cobalt families.1 In addition, mercury-titanium bonded species have been postulated as unstable reaction intermediates.2... 

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