Chromium stability constant

The aqueous chemistry of chromium(in) above 100 °C has been investigated with particular reference to corrosion phenomena and the possibility of hydro-thermal synthesis of chromite in serpentine rocks. Isotopic exchange studies have indicated that the CrO unit exchanges intact between [Cr(H20)6] and [CrMo6024H6] . This appears to be the first established exchange of such a unit. ° [CrlHjOljlEtOH)] has a stability constant of 6.5 x 10" and is only stable in perchlorate solutions with >80% EtOH. ... 

Trihydroxyglutaric acid forms a 1 1 complex with chromium(iii) which has a stability constant of 0.9 x 10. In the presence of copper(n) a mixed complex is formed and dark bluish-green crystals of Na3[CuCrCioH70i4],5H20 have been isolated. The substituted benzoic acid complexes (60) have been prepared by heating a mixture of CrCL,6H20 and RCgH4C02H in Pr OH. The... 

Table 39 contains some stability constant data for chromium(lI) and amino acids in aqueous... 

The chromium(II)-edta system is powerfully reducing (the half-wave potential is -1.48 V at pH 12 vs. SCE) and has been used in the reduction of iron-sulfur clusters.292 No solid complex has been isolated because of its instability to oxidation, but Cru-edta is high-spin in aqueous solution (/ieff = 5.12 BM) and its stability constant has been determined. The edta is believed to be pentadentate with H20 in the sixth position.293... 

Table 39 Stability Constants of Chromium(II) Complexes in Aqueous Solution...

Stability constants for higher oligomers have, until now, been reported only for the aqua chromium(III) system. In addition to Bjerrum s classical study of this system (14), stability data for this very complicated system have been reported by several independent groups (26,27, 28, 200). The Qxy values given in Table XIV show some deviation between the values reported by different authors, and it should further be noted that ( 46 applies to a mixture of at least two tetranuclear species (see Section IV). The stability constants Kn for the stepwise polymerization process, Eq. (32), have been determined as K2 1 x 105 M 1, K3 x6 x 106 M l, and K x 2 x 105 M The change in free... 

E.s.r. spectra have been reported114 for several oxalato-chromium(m) complexes and the thermal decomposition products of Cr2(C204)3,6H20 characterized by X-ray powder photography.115 The stability constants for 1 1. 1 2. and 1 3 chromium(m) formato-116 and tartrato-117 complexes have been determined, and the constitution and charge of 1 1 and 1 2 chromium(m) complexes with aspartic acid and asparagine reported.118... 

The thermal decomposition of chromium(iii) oxalato-complexes has been shown to proceed via electron transfer and C—O bond fission, to afford CO2 and a smaller quantity of The stability constants of some chromium(m)... 

JV-Mercaptoacetamidophenol (59) complexes with chromium(iii) to afford a 1 A complex with a stability constant of 5.4 x 10 lmol The chromium-... 

Table 1-6. Stability constants for complexes of iron, chromium, and nickel ions with halides .

Due to the stability of intermediate complexes between the metal substrate and the aggressive anions, pitting corrosion does not occur for chromium metal. Stability constants of CrX complexes are smaller than 1, for instance it is 1 when X is C1 and lO" when it is 1. In addition, exchange of CT and HjO ligands between the inner and outer sphere of chromium halide complexes is extremely slow. Together these factors causes insolubility of CrClj in cold water due to very low dissolution rate of Cr. Therefore the presence of a Cr-Cl complex at the surface will not increase the dissolution rate because it will dissolve very slowly by itself. In the case of this exchange is very rapid. Similarly Fe-Cr alloys are more resistant to pitting in Cl" solution than is pure Fe. 

Whereas the assignment of mechanism to spontaneous thermal aquations may at times be uncertain, the mechanism of metal-ion-catalysed aquation of halide complexes of cobalt(iii), chromium(ni), and similar cations is unlikely to be other than dissociative as far as the metal(m) centre is concerned. In Volume 2 of this Report it was mentioned that the catalytic effect of metal ions on solvolysis rates of t-butyl halides could be correlated with the stability constants of the respective metal-halide complex formed. Such a correlation is now reported for metal-ion catalysis of aquation of halide complexes of cobalt(m), chromium(m), and rhodium(m). Indeed this correlation is sufficiently general as to embrace such catalysts as H+ and HgCl+ as well as metal ions such as Hg + and A linear free-energy (AG vs. AG°) correlation... 

Catalysed Aquation.— In 1971 Rudakov and Kozhevnikov reported a correlation between the rate constants for metal-ion (M +) catalysed solvolysis of t-butyl halides and the stability constants of the respective complexes Now these authors have shown that a similar correlation applies, albeit rather approximately, to metal-ion catalysis of aquation of halogeno-cobalt(ra), -chromium(ni), and -rhodium(in) complexes. In fact the catalysts mentioned include not only metal ions such as Hg , Tl +, and Ag+, but also complexes such as HgCl+ and T1C1 +. This correlation can be improved by making an allowance for the different coulombic repulsions in systems of different charge products. If the rate constant for the catalysed aquation is ki and that for uncatalysed aquation Ato, the stability constant of the metal-ion complex produced K, the product of the charges on the reactants zazb, and C is a coulombic interaction constant, then the correlation conforms to the equation... 

Activation parameters for the path corresponding to the A i[Fe +]term are — 24.2 0.5 kcal mol and IsS — 8.9 1.6caldeg mol The iron(m) assists the oxalate group to leave by co-ordinating to it, possibly in a unidentate-oxalate intermediate rather than in the starting complex itself. Several examples of metal-ion-catalysed aquation of chromium(ra) complexes are included in the general rate constant against stability constant correlation discussed in the section on catalysed aquation of cobalt(m) complexes. 

Af2 = 0.55 0.18 kg mol" because the data available for CrOH" only extend to 1.05 molkg-. As can be seen from Figure 11.24, the fit to the data is quite reasonable using this value of Ae2- The stability constant accepted for CrOH, at 25 °C and zero ionic strength, is in excellent agreement with that selected by Ball and Nordstrom (1998) (log = -3.57 0.08) in a review on the thermochemistry of chromium, its oxide and hydroxide species and phases. 

The solubility of chromium hydroxide has been studied by Ziemniak, Jones and Combs (1998) and Rai et al. (2002, 2004). These studies do not provide sufficient information to be able to derive data for the present review. However, Rai et al. (2004) provide data for the formation of Cr(OH)3 (aq) from CrOH and CrfOH) " from CrfOHfgfaq), at zero ionic strength, from both studies. StabiUty constants (logic") calculated for the first reaction were —10.93 and —10.66, respectively. Neither value is consistent with the value of log K° = -12.65 derived in the present review. For the second reaction, the stability constants derived were -11.52 and -12.93, respectively. The value from Rai et al. (2004) is in quite good agreement with that determined in the present review (log K = -11.31). Rai et al. (2002) also postulated the species Cr202(0H)4 , but because there is no confirmatory evidence for this species, it has not been retained in the present review. 

Stability constants for a number of polymeric hydrolysis species of chromium(III) have been determined due, in part, to the slow kinetics of formation of these species. The most important appear to be Cr2(OH)2 and CrgfOH) for which data are available from more than one laboratory. These two species are considered reliable in this review and the available stability constant data are listed in Table 11.16. 

The dominant valency state of molybdenum in aqueous solution is +6, although at reduced both the +3 and +4 states can form. Hexavalent molybdenum is anionic, whereas no hydrolysis data have been reported for molybde-num(lV). Molybdenum(III) is cationic and should behave in a similar fashion to chromium(III). The ionic radius for sbc-coordinate Mo has been reported by Shannon (1976) to be 0.69 A. Stability constants for the monomeric hydrolysis species of molybdenum(III) have been reported. 

Mel chakova and Peshkova (1978) assumed that the protolysis constant of water for the conditions used was log = -14.0. This value is somewhat different from that derived in the present review for the conditions studied, that is, log =-13.88. This difference indicates that the stability constants proposed should be more positive than indicated by 0.12 log units per OH molecule in each proposed species. This suggests that the stability of molybdenum(lll) hydrolysis species would be substantially more stable than those of chromium(lll). This is considered unlikely on the basis of the corresponding ionic radii of the two ions. Molybdenum(III) has a larger ionic radius than chromium(III) (Shannon, 1976) and, as such, would likely have hydrolysis species of lesser stability. Thus, the stability constants listed by Mit kina, Mel chakova and Peshkova (1978) are not retained (but see Chapter 16). 

Mit kina, Mel chakova and Peshkova (1978) obtained stability constants for the first three monomeric hydrolysis species of molybdenum(III). The stability constants proposed were log = -2.0 0.1, log 2 = 4.6 0.1 and log 3 = -7.3 0.1 at 20 C and in l.OmolH (Na,H)Cl. Use of the UTMIC to predict the stability constants of these species (gj z/r +g2) = S6.01 assuming n D)=0) leads to values of log = -2.41, log 2° = -5.89 and log 3° = -11.13, respectively. These values are less positive and appear to be inconsistent with the values of Mit kina, Mel chakova and Peshkova (1978) confirming the decision not to retain the values. However, the UTMIC does demonstrate that the stability constants of molybdenum(III) should be more stable than the equivalent species of chromium (III). 

---