Chromium coordination number

Chromium (II) also forms sulfides and oxides. Chromium (II) oxide [12018-00-7], CrO, has two forms a black pyrophoric powder produced from the action of nitric acid on chromium amalgam, and a hexagonal brown-red crystal made from reduction of Cr202 by hydrogen ia molten sodium fluoride (32). Chromium (II) sulfide [12018-06-3], CrS, can be prepared upon heating equimolar quantities of pure Cr metal and pure S ia a small, evacuated, sealed quartz tube at 1000°C for at least 24 hours. The reaction is not quantitative (33). The sulfide has a coordination number of six and displays a distorted octahedral geometry (34). 

Ceitain acid dyes can have thek fastness piopeities impioved by combining the dye with a metal atom (chelation). The most common metal is chromium, although cobalt is sometimes used, and this can be introduced in a number of ways. The basic mechanism is donation of electron pans by groups in the dye (ligands) to a metal ion. For example, has a coordination number of 6, and therefore will accept six lone pans of electrons. Typical ligand groups... 

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

If more than simple atoms are bound to a central atom, then the coordination number still refers to the number of near neighbors. For example, in solid potassium chrome alum, KCr(S04)2- 12H20, and also in its fresh aqueous solutions, the chromium-containing cation is... 

SCN- is the thiocyanate ion). Consider the oxidation number of chromium to be +3 and the coordination number to be 6 in both compounds. Estimate... 

Among the compounds that form complexes with silver and other metals are benzene (represented as in 9) and cyclooctatetraene. When the metal involved has a coordination number >1, more than one donor molecule participates. In many cases, this extra electron density comes from CO groups, which in these eomplexes are called carbonyl groups. Thus, benzene-chromium tricarbonyl (10) is a stable compound. Three arrows are shown, since all three aromatic bonding orbitals contribute some electron density to the metal. Metallocenes (p. 53) may be considered a special case of this type of complex, although the bonding in metallocenes is much stronger. 

The reactivity of chromium(V) and chromium(IV) species is uncertain since there are no reliable thermodynamic data, and not much can be said at present about the structure of these species. With respect to the latter some hints can be obtained from the fact that the changeover from chromium(V) to chromium(IV) or vice versa in all cases was found to be rate determining, which seems to correlate well with the conclusion of Tong and King d that Cr(VI) and Cr(V) have coordination number four, whereas Cr(IV) and Cr(III) have six. 

Much of what has been said so far in this chapter applies equally well to complexes of second- and third-row transition metals. However, there are some general differences that result from the fact that atoms and ions of the second- and third-row metals are larger in size than those of first-row metals. For example, because of their larger size (when in the same oxidation state as a first-row ion), ions of metals in the second and third rows form many more complexes in which they have a coordination number greater than 6. Whereas chromium usually has a coordination number of 6, molybdenum forms [Mo(CN)8]4 and other complexes in which the coordination number is 8. Other complexes of second- and third-row metals exhibit coordination numbers of 7 and 9. 

The slow acetylation of the hydroxyl group is difficult to explain. One is inclined to suggest that this group is coordinated to the metal ion and consequently rendered inactive. Such a possibility requires either a coordination number of 7 for chromium (III) or displacement of carboxylate from the coordination sphere by hydroxyl. In the latter instance an uncoordinated functional group would still be present to react with ketene (COO or COOH) and anhydride should be detected in the crude reaction product. This is not the case. 

A coordination number of 7 does not seem likely for chromium (III) also, the infrared spectrum indicates that this compound contains uncoordinated hydroxyl groups. The similarity of the visible-ultraviolet absorption spectra of [Cr(HO-A)2] and [Cr(AcO-A)2] (above) is further evidence of the identical character of the donor groups in both compounds, and hence, hydroxyl groups appear to be uncoordinated in the former. We must seek an explanation not involving coordination of the hydroxyl oxygen to chromium (III). 

This section is dominated by dialkyl- and disilyl-amide ligands, which, because of steric factors, constrain chromium(III) to the unusual coordination numbers of three or four. 

It is often useful to consider that sites for chemisorption result from surface coordinative unsaturation, i.e., that atoms at the surface have a lower coordination number than those in bulk. Thus, for example a chromium ion at the surface of chromium oxide has a coordination number less than that of a chromium ion in the bulk. The chromium ion will tend to bind a suitable adsorptive so as to restore its coordination number. An atom in the (100) surface of a face-centered cubic metal has a coordination number of 8 vs 12 for an atom in bulk this, too, represents surface coordinative unsaturation. However, of course, there are sites to which the concept of surface coordinative unsaturation does not apply, for example, Br nsted acid sites. 

CrX3 ]X (aq.). Trivalent chromium salts form several distinct kinds of solutions the purple solution, which is supposed to contain [Cr 6H20]X3 the green solution, which is supposed to contain [Cr 4 H20 X2]X and others of less importance for the present purpose. In writing the formulas of the aqueous ions and molecules, we have omitted the water molecules, so that for the above molecules are written [Cr]X3 and [Cr X2]X and the positive ions [Cr]+++ and [Cr X2]+. The number of molecules of H20 necessary to make the coordination number of chromium have the value 6 have been omitted in writing the formulas... 

The coordination number of the chelated metal atom determines the number of linkages to functional groups. It is typically greater than the valency of the metal ion for example, the divalent ions of copper and nickel have coordination numbers of four, and the trivalent ions of chromium, cobalt, iron a coordination number of six. In the case of iron the coordination number six applies for the di- and trivalent forms. 

CN depends not only on the composition of a coordination compound, but also on the type of a- and n-bonds present in it. Chromium carbonyl-pyridine complexes are the classic example to illustrate the difficult task of determining unambiguously the coordination numbers. Only a-bonds are present in Cr(py)(CO)5 and CN = 6 (12). However, in the a, n-complex 13, as well as in 9, the coordination number is quite questionable (compare formulae 9 IT). 

Chromium ions at the surfaces of inorganic oxides are characterized by a wide variability of the oxidation state, coordination number, and local structure. Consequently, Cr-based materials are especially attractive as catalysts. Much is known about the catalytic activity of pure Cr203 for various reactions (469), including polymerization of alkenes (470-472), hydrogenation-dehydrogenation of hydrocarbons (473-481), reduction of NO and decomposition of N2O4 (482), and oxidation of organic compounds (483, 484). 

LaCrC>3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC>3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

Until now we have been dealing with metals of coordination number 4 which normally lead to linear type polymers. When trivalent chromium... 

There seems to be even less structural similarity for many other metal halides as the crystalline systems are compared with the molecules in the vapor phase. Aluminum trichloride, e.g., crystallizes in a hexagonal layer structure. Upon melting, and then, upon evaporation at relatively low temperatures, dimeric molecules are formed. At higher temperatures they dissociate into monomers (Figure 9-58) [107], The coordination number decreases from 6 to 4 and then to 3 in this process. However, at closer scrutiny, even the dimeric aluminum trichloride molecules can be derived from the crystal structure. Figure 9-59 shows another representation of crystalline aluminum trichloride which facilitates the identification of the dimeric units. A further example is chromium dichloride illustrated in Figure 9-60. The small oligomers in its vapor have structures [108] that are closely related to the solid structure [109], Correlation between the molecular composition of the vapor and their source crystal has been established for some metal halides [110],... 

With these aluminium halides still other complications occur, because the coordination number amounts to four and not three as is still the case for boron. The molecules consist in the gas phase, as in the liquid phase, of molecules A12X3, two tetrahedra with a common edge. These molecules are also present in the solid state, at least for the bromide and iodide, not however in the chloride, which still possesses a coordination lattice similar to that of chromium chloride (Ketelaar, MagGillavry and Renes). Aluminium chloride thus just forms the transition. 

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