Complexes of Chromium III

The malonato complexes of chromium(III) are analogous to the oxalate complexes of chromium(III). Since malonic acid is a weaker acid than oxalic acid, the malonato complexes are expected to be more labile than the oxalato complexes. The dicarboxylate complexes of chromium(III) form a group of anionic complexes which are suitable for the study of octahedral complex reactivity. 

Aside from its color, trivalent chromium differs most markedly from Al(III) in its complexing ability. There are no analogs in aluminum chemistry for many of the octahedral complexes formed by Cr(III) with CN, SCN, NOj, NH3, and with a number of organic amines. The octahedral complexes of aluminum are confined to those having Al—0 or Al—F bonds. In contrasting the complexes derived from the two 

It has recently been shown that oxidation of Cr+2 by such one-electron oxidizing agents as Fe(III) and Cu(II), yields Cr(H20) fs or one of its substitution products, whereas oxidation with such two electron oxidizing agents as HCIO and H202 yields a dinuclear chromium complex, probably (H20)5Cr—0—Cr(H20) 4. This difference in behavior 

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As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Methods for the preparation of tris(0-ethyl dithiocarbonato) complexes of chromium(III), indium(III), and cobalt(III) are presented and serve to illustrate procedures applicable to the preparation of O-alkyl dithiocarbonato, alkyl trithiocarbonato, iV,A7-dialkyldithiocarbamato, and 0,0 -dialkyl dithiophosphato complexes of several metals. 

TABLE I Solubilities of Tris(0-ethyl dithiocarbonato) Complexes of Chromium(III), Indium(III), and Cobalt(III) in Organic Solvents at 25°... 

A review of recent advances in chromium chemistry (82) supplements earlier comprehensive reviews of kinetics and mechanisms of substitution in chromium(III) complexes (83). This recent review tabulates kinetic parameters for base hydrolysis of some Cr(III) complexes, mentions mechanisms of formation of polynuclear Cr(III) species, and discusses current views on the question of the mechanism(s) of such reactions. It seems that both CB (conjugate base) and SVj2 mechanisms operate, depending on the situation. The important role played by ionpairing in base hydrolysis of macrocyclic complexes of chromium(III) has been stressed. This is evidenced by the observed order, greater... 

The unfilled sites in 1 1 metal-dye complexes of chromium(III) or cobalt(III) ions can be... 

Thermally, ammine complexes of chromium(III) containing a coordinated ligand X (where X is CL, CNS , etc.) undergo substitution of X by H20 in aqueous solution with retention of stereochemistry ... 

The thio- and seleno-phosphinato-complexes of chromium(iii), [Cr Se-(S)PR2 3] (R = Et or Ph) and [Cr Se(Se)PEt2 3], have been prepared by treating anhydrous or hydrated CrCl3 with the corresponding NaSe(X)PR2 salt, and characterized by their electronic and i.r. spectra. ... 

Triethanolamine complexes of chromium(iii) have been reported and characterized in i.r. spectra and thermal-decomposition studies. The chromium(iii) nitrilotriacetato-complex [CrL(H20)2] [L = N(CH2C02)3 ] complexes with thallium(iii) to give [ CrL3(0H)(H20) gTl] with a formation constant of 9 3 X 10 at 25 The hydrolysis and dimerization of [CrLlOHljV ... 

Some recent advances are the isolation of aryl isocyanide complexes of chromium(III) Cr(CNAr)6]X3, and six- and seven-coordinate alkyl isocyanide complexes of chromium(II) Cr(CNR)6j7]X2. All but the chromium(O) and the seven-coordinate chromium(II) complexes lave less than 18 electrons in the valency shell. 

The coordination chemistry of chromium(III) was first seriously investigated by Pfeiffer at the turn of the century in many ways his studies parallel the work of Werner on cobalt(III). The complexes of chromium(III) are almost exclusively six-coordinate with an octahedral disposition of the ligands. Many are monomeric ((Jeff 3.6 BM), although hydroxy-bridged and other polynuclear complexes are known in which spins on neighbouring chromiums are coupled. 

The reported complexes of chromium(III) with group IV ligands are limited mainly to those containing CN and to a lesser extent those containing other C donor ligands. 

Complexes of chromium(III) with ammonia and amines were among the first to be recognized by the early coordination chemists. Consequently, there is a vast literature, and this section is restricted to work since the comprehensive review by Garner and House302 in which... 

The chemistry of amine complexes of chromium(III) up to 1969 has been extensively reviewed by Garner and House 302 consequently earlier work will not be covered here. Fluorodiamine complexes of chromium(III) have also been carefully reviewed by Vaughn.402,403... 

Complexes of chromium(III) with multidentate ligands containing azo linkages as well as other donor groups are of interest because of their applications in spectrophotometric analysis and because of their uses as commercial dyestuffs. Some ligands used in the analysis of chromium(III) are listed in Table 65. 

Despite the vast interest shown by coordination chemists in polypyrazolylborate ligands (17) since their discovery in 1966 few complexes of chromium(III) with these ligands have been mentioned in the literature. The complex [ B(pz)4 2Cr]PF6 (147) may be prepared by either of the routes outlined in Scheme 89.657... 

Camphorate complexes of chromium (III) have been studied. The four possible isomers of the tris complex of (+ )-3-acetylcamphorate (173) were isolated,752,753 and absolute configurations were tentatively assigned. The photoisomerization of these complexes has been investigated 754 quantum yields of the order of 10-3 were obtained with visible or ultraviolet radiation at temperatures around 100 °C. Bond-breaking processes were held to be important in the reactivity of cis isomers. 

Trifluoromethylsulfate forms weak complexes with chromium(III) 821 this anion may provide a viable alternative to perchlorate as a non-coordinating anion (for Cr(03SCF3)3 see Section 35.7.2). Tris(0,0 -sulfinate) complexes of chromium(III) have been prepared (RSO2-, R = Me, C6H5, p-CH3C6H4) typical monomeric complexes are obtained.822... 

Simple carboxylates of chromium(III) find industrial application as catalysts for the polymerization of to-alkenes854,855 and in the preparation of chrome-tanning solutions.856,857 There seems to be no simple carbonate of chromium(III). Compounds formed on the surface of Cr203, sometimes formulated Cr2(C03)3- H20, are best viewed as carbon dioxide adsorbed on the oxide.858 Carboxylate complexes of chromium(III) will now be considered in terms of the various ligand types. 

Oxalate complexes of chromium(III) were first characterized at the turn of the century by Rosenheim and Cohen.877 The most extensively studied are the tris species and the cis- and frarw-bisoxalates (205-207) these formulations were first suggested by Werner.878 All may be made by the reduction of chromate with oxalate. Reliable preparations have been reported for tris by Kauffman and Faoro879 and for cis- and rrans-diaquabisoxalatochromateflll) by Bailar.880... 

An isolated report of the synthesis and characterization of a number of citrate complexes of chromium(III) has appeared.938... 

Complexes of salicylate with chromium(III) have not been reported but the tris complexes of salicylaldehyde and chromium(III) may be prepared by refluxing [Cr(THF)3Cl3] with salicylal-dehyde and sodium acetate in ethanol.939 The acid hydrolysis of this complex was studied in detail, but the isomerism obviously possible for this complex was not apparently considered. Khan and Tyagi940 studied the formation of phthalate complexes of chromium(III). 

An extensive, if somewhat dated, review of the coordination chemistry of aromatic TV-oxides is available.954 Most studies of the A-oxide complexes of chromium(III) involve the preparation and characterization (usually by magnetic and spectroscopic methods) of monomeric or hydroxy-bridged complexes and are summarized in Table 84. 

The formations of chloro,1087,1088 bromo1089 and iodo1090 complexes have been studied. Recently, electrochemical methods have been used to measure equilibrium constants for the formation of chloro (0.086), bromo (1.1 x 10 3) and iodo (1.1 x 1(T5) complexes of chromium(III).1091... 

The complexes of the monothio-/3-diketonate RC(SH)=CHCOR (R = R = phenyl R = phenyl, 2-thienyl, j8-naphthyl R = CF3), which are acetylacetonate analogues have been synthesized (242).1143 Dipole moment measurements are consistent with facial structures (243). Mass spectra indicate no metal-containing peaks for the complex [Cr(PhC(S)=CHCOPh)3)] however, for the fluorinated monothio-j8-diketonates, various metal-containing peaks, e.g. M—2b=F, were observed. Such ions involve fluoride migration. Monothiooxalate complexes of chromium(III) have been prepared fairly unusual complexes, exemplified by [Cr (C2S03)Cu(Ph3)2 3], were reported. On refluxing under chloroform (7h) reactions of the kind illustrated were alleged to occur (equation 55). 44... 

Complexation of chromium(III) by nitrilotriacetic add (H2nta) has been investigated. Values of equilibrium constants (/ = 0, 25 °C) are log K = 10.66 and log K2 — 8.73.1234 A large number of mixed complexes with nta/chromium(III) have been prepared, including mixed aminoad-dates,1235 acetylacetonates, catecholates, oxalates and complexes with 1,10-phenanthroline and 2,2 -bipyridyl.1236 1238... 

HYDROXO-BRIDGED COMPLEXES OF CHROMIUM(III), COBALT(III), RHODIUM(III), AND IRIDIUM(III)... 

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