Formation of Chromium III Complexes

The formation of K3[Cr(CN)sOH] is reported from the reaction of KCN with [Cr(NH3)5Cl]Cl2 followed by a Sephadex gel column separation.  

The formal end member from the deprotonation of [Cr(OH2)6] is [Cr(OH)6]. This has now been prepared as a well crystalline blue-green solid (Section 6.8.1, No. 8) using the fact that NO3 ions react with NH2 ions in NH3 to give OH as in equation (10). 

A detailed time-dependent product distribution of the hydrolytic oligomers formed when up to 0.8 equivalents of base is added to Cr (aq) is now available.  

After the addition of base, oligomerization causes a relatively rapid (hours) drop in pH. The concentration of the dimer (2) reaches a maximum after a few days (/ = 1.0 M, NaC104, T - 25.0 °C) while the trimer (3) concentration increases continuously throughout the reaction and finally becomes the dominant product. The tetramer (4) concentration eventually (months) becomes constant and the 

Substitution Reactions of Inert Metal Complexes—6 and Above 

Unlike the studies of [Cr(H20)6] reacting with carboxylate ions, the reaction with 1,10-phenonthroline in aqueous ethanol is reported to be independent of pH in the range 3.5-5.0. A simple second-order rate law was observed. However, for the reaction with nitrilotri-acetate (nta) ions in the same solvent mixture, A obs = A E[ntaH ]/(l -f Ar [ntaH ]), where is the ion pair formation constant.An mechanism is favored. 

Oxo anions are known to catalyze loss of ligands from aquo-chromium(III) complexes as discussed in Section 5.2.2.1.2. Two groups have studied the reaction of Cr(III) with xylenol orange, in the presence and absence of 

Reaction of [Cr(H20)6] with thio-bis(ethylene nitrilo)tetraacetic acid (TEDTA) gives a violet complex 3 in which TEDTA is quinquedentate. At pH 

3 was observed to rearrange to a pink complex 4 in which TEDTA is sexidentate. 3 was also found to dimerize in the presence of base to give 5, which then reacted further to give the green product 

10-phenanthroline has been reexamined by stopped-flow spectrophotometry. The data for the loss of one bipy or phen molecule are given in Table Data for the thermal aquation of [Cr(phen)3] are 

Several studies of the reactions of [Cr(H20)6] and [Cr(H20)50H] with amino acids and related ligands have appeared. The reaction of [Cr(H20)sOH] with d/-alanine has been studied between pH 4.5 and 5.4 

The interesting complex ion [CrFsOH2] is reported to be kinetically inert. It is formed by reduction of CrOs by formaldehyde in 40% aqueous Hf.( 4) 

Formation of metastable [Cr(H20)5L] ions is often achieved by inner-sphere redox reactions, usually by the reaction of [Cr(H20)6] with inert-metal complexes such as [Co(NH3)5L] . Recent examples include cases where L = tetrazoles, 0S02NH2, 3- or 4-CNC6H4CO2 and [0CC(CH3)C(CN)C(CH3,C0].  

The chromium(III) ion is a common crosslinker for preparing profile control gels for water flow control in oil reservoirs. This practical application of chromium chemistry has lead to two studies on the rate and mechanism of gel formation. A preliminary report using H NMR relaxation 

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The formation of chromium(iii) complexes of ethanolamine-NN-diacetic acid in aqueous solution has been investigated by potentiometric and colorimetric methods. A1 1 complex is found to be formed in the pH range 3—6. At pH > 6 hydrolysis takes place. Isolation of mono-, di-, and hetero-nuclear Cr chelates of ttha (ttha = triethylaminetetra-aminehexa-acetic acid) has been accomplished. No evidence for exchange coupling was found for either the dinuclear complex or for the heteronuclear Cr, Cu complex, CrCu(Httha)2,7H20. ... 

The formation of a number of chromium(III) complexes Cr(H20)5 from their constituent ions Cr(H20)g and X ,... 

The easy formation of hydroxo- or oxo-bridged Cr111 polymers in basic aqueous solution, the comparative lability of the Cr—N bond, and the precautions needed to obtain chromium(II) complexes compared with cobalt(II) complexes have meant that the preparative chemistry of chromium(III) is more difficult than that of cobalt(III). A greater variety of non-aqueous solvents is now in use, and there is greater knowledge of chromium(II) chemistry to be exploited in the preparation of chromium(III) complexes generally, but few new methods of preparation of amine complexes have been devised since the early work. 

The traditional preparation of chromium(III) complexes involves reacting the brilliant blue ion with phenanthroline ligand. In this experiment we will prepare [Cr(phen)3]3+, Figure 8.1, using a revised method in which the precipitation of AgCI(s) drives the complex formation and eliminates the need for inert atmosphere conditions. 

Polarographic reduction of chromium(iii) complexes of dithiocarbomates R2NCS J (R = Et, Mr, or Bu") and of heterocyclic amines of the type (CH2)2NCSJ (n = 4,5, or 6) in the presence and absence of the ligands bipy and phen has been studied in DMF. The addition of the first electron occurs reversibly in all cases and the rate of formation of adducts with phen is greater than with bipy. Electrolysis of a solution of Cr(Et2 NCS2)3 in MeCN at — 1.1 V in the presence of bipy gives rise to the sequence of reactions shown in Scheme 4. The Cr product species was identified by e.s.r. spectroscopy. Normal co-ordinate analysis on a series of chromiumfiii) dithiocarbamates... 

Several reports concern the formation of organic ligand complexes of iron(iii). The variation of rate with ligand concentration in the reaction with mandelic acid > is interpreted as a pre-equilibrium ion-pair formation followed by dissociative complex formation within the ion-pair, rather than as simple 5 n2 formation. This interpretation is similar to that proffered for formation of malonate and oxalate complexes of chromium(iii) (see above). Rates of reaction of iron(iii) with a variety of phenols are all very similar, suggesting that iron(iii)-water bond breaking is rate determining here also. Sulphosalicylate reacts with FeOH + by the same rate-determining loss of water from the iron(m). Rates of formation of iron(iii) complexes with acetate, monochloroacetate, and propionate have been reported. ... 

Standard oxidation potential for complexes [Cr(arene)(CO)3] in CF3COOH + (CF3C0)20 (93 + 7%) solution is a linear function of the ionization potentials of arene hydrocarbons.The oxidation in trifluoroacetic acid is reversible, while in MeCN and DMF solutions it is irreversible.Further oxidation leads to decomposition of these complexes and the formation of chromium(III) compounds and separation of ligands. 

Ligand field irradiation of chromium(III) complexes leads primarily to substitution reactions/ The most common reaction in aqueous solution is photosubstitution of a ligand by water. One of the earliest studies of photoaquation reactions was the light-induced exchange of water between Cr(H20) and the solvent. The reaction is followed by using isotopically labeled H2O as the solvent, where sequential photosubstitution of the H2O molecules leads to the formation of Cr(H20 )6 (Ref.3) ... 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

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... 

Chromium(III) catalyses the cerium(IV) oxidation of primary and secondary alcohols in a mixture of H2SO4 and HC104. Kinetic results have been interpreted in terms of the formation of chromium(IV) in a reversible equilibrium, which forms a complex with the alcohol. Internal oxidation-reduction occurs in a rate-determining step to give aldehyde or ketone and regenerate the catalyst in the +3 state. The oxidation of ethanol under similar conditions has also been studied. ... 

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 ... 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

Electro-generated chromium(II) is also very effective in the pinacolization of otherwise unsatisfactory dimerizing carbonyl compounds (Table 5, No. 13)220-222) this case the chromium(II) ion does not act as redox agent but catalyzes the formation of a chromium(III) complex of the carbonyl compound which subsequently is reduced to the pinacol directly at the cathode (Eqs. (73)-(77)). 

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