Chromium coordination compounds

Four different octahedral chromium coordination compounds exist that all have the same oxidation state for chromium and have H2O and Cr as the ligands and counterions. When 1 mol of each of the four compounds is dissolved in water, how many mol of silver chloride will precipitate upon addition of excess AgNOj ... 

A student in 1895 prepared three chromium coordination compounds having the same formulas of CrCl3(H20)6 with these properties ... 

Chromium(III) Chemistry. The most characteristic reactions of Cr(III) in aqueous solution at >4 pH, eg, in the intestine and blood, and hydrolysis and olation (147). As a consequence, inorganic polymeric molecules form that probably are not able to diffuse through membranes. This may be prevented by ligands capable of competing for coordination sites on Cr(III) (see Coordination compounds) (147). Thus any large fraction of ingested Cr(III) should be absorbed. Chromium (ITT) in the form of GTF may be more efficiendy absorbed. 

Ammonia forms a great variety of addition or coordination compounds (qv), also called ammoniates, ia analogy with hydrates. Thus CaCl2 bNH and CuSO TNH are comparable to CaCl2 6H20 and CuSO 4H20, respectively, and, when regarded as coordination compounds, are called ammines and written as complexes, eg, [Cu(NH2)4]S04. The solubiHty ia water of such compounds is often quite different from the solubiHty of the parent salts. For example, silver chloride, AgQ., is almost iasoluble ia water, whereas [Ag(NH2)2]Cl is readily soluble. Thus silver chloride dissolves ia aqueous ammonia. Similar reactions take place with other water iasoluble silver and copper salts. Many ammines can be obtained ia a crystalline form, particularly those of cobalt, chromium, and platinum. 

Most mordant dyes are monoazo stmctures. The most important feature of this class of dyes is excellent fastness to light and washing. Mordant dyes are available ia aU shades of the spectmm with the exceptioa of bright violets, blues, and greens. To be useful, the metal complexes must be stable, ie, must not demetallize when subjected to dyebath conditions and aU aftertreatment processes, especially repeated washings. Chromium forms stable chelate rings with mordant dyes which are not affected by treatment with either weak acid or alkaU (see Coordination compounds). 

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

Leather Tanning and Textiles. Although chromium (VT) compounds are the most important commercially, the bulk of the appHcations in the textile and tanning industries depend on the abiUty of Cr(III) to form stable complexes with proteins, ceUulosic materials, dyestuffs, and various synthetic polymers. The chemistry is complex and not well understood in many cases, but a common denominator is the coordinating abiUty of chromium (ITT) (see LEATHER Textiles). 

Na[AuClJ, per mole of silver haHde. Coordination compounds are used as emulsion stabilizers, developers, and are formed with the weU-known thiosulfate fixers. Silver haHde diffusion transfer processes and silver image stabilization also make use of coordination phenomena. A number of copper and chromium azo dyes have found use in diffusion transfer systems developed by Polaroid (see Color photography, instant). Coordination compounds are also important in a number of commercial photothermography and electrophotography (qv) appHcations as weU as in the classic iron cyano blueprint images, a number of chromium systems, etc (32). 

Chromium atoms were cocondensed with benzyl sulfide at 77 K (35), the primary result being desulfurization to form bibenzyl and trans-stilbene. Coordination compounds were not characterized in this system. 

The two most common series of chromium halides have the formulas CrX2 and CrX3 (where X = F, Cl, Br, or I). However, CrF6 is also known. Compounds having the formula CrX3 are Lewis acids, and they also form many coordination compounds. For example, CrX3 reacts with liquid ammonia to yield... 

In reactions involving coordination compounds, the metal acts as the Lewis acid (electron-pair acceptor), while the ligand acts as a Lewis base (electron-pair donor). In the reaction above, the ammonia ligand displaced the water ligand from the chromium complex because nitrogen is a better electron-pair donor (less electronegative) than oxygen. 

Forms a number of coordination compounds (ammonia complex) with several metals adds to AgCl forming soluble complex [Ag(NH3)2]Cl forms tetraamine complex [Cu(NH3)4]S04 with CUSO4 and forms many hexaamine complexes with cobalt, chromium, palladium, platinum and other metals. 

The industrial and biochemical importance of coordination compounds of chromium is described in Volume 6, and aspects of theoretical, mechanistic and solution chemistry are discussed in Volume 1 of this series. 

Chromium(IV) does not have any aqueous solution chemistry except for the formation of intermediates in the reduction of CrVI to Crm. Chromium(IV) compounds tend to disproportionate into Cr111 and CrVI species (equation 78) and the metal ion in this oxidation state is powerfully oxidizing towards organic compounds. An eight-coordinate complex [CrH4(dmpe)4] is known (Section 35.3.4.1). 

Chromium coordinates selectively with the 1,2-diol, forming a stable cyclic chromate ester that evolves producing the formation of a tetrahydrofuran. Observe that no formation of tetrahydrofuran from the alcohol on the left occurs, for this would involve the intermediacy of a less stable simple chromate ester (vide infra). The experimental conditions are so mild that no direct oxidation of the secondary alcohol to ketone is observed, either on the starting compound or in the product. 

Kane-Maguire NAP (2007) Photochemistry and Photophysics of Coordination Compounds Chromium. 280 37-67 Kann N, see Ljungdahl N (2007) 278 89-134... 

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). 

Whole-body analysis of mice given a single intraperitoneal injection of 3.25 mg chromium(III)/kg as chromium trichloride showed that chromium trichloride was released very slowly over 21 days 87% was retained 3 days after treatment, 73% after 7 days treatment, and 45% after 21 days. In contrast, mice given a single intraperitoneal injection of 3.23 chromium(VI)/kg as potassium dichromate retained only 31% of the chromium(VI) dose at 3 days, 16% at 7 days, and 7.5% at 21 days. Mice injected weekly with chromium(III) compounds at 17% of the LD50 retained 6 times the amount of chromium as mice injected with chromium(VI) compounds at 17% of the LD50. The retention of chromium(III) was attributed to its ability to form coordination complexes with tissue components such a proteins and amino acids (Bryson and Goodall 1983). 

The overwhelming majority of benzannulations with chromium carbene complexes involve compounds in which the metal bears only carbonyl ligands. However, phosphine ligands may also be present in the chromium coordination sphere. 

Kane-Maguire NAP. Photochemistry and photophysics of coordination compounds Chromium. In Balzani V, Campagna S, eds. Photochemistry and Photophysics of Coordination Compounds I. Berlin Springer, 2007 37-67. 

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