Cobalt hydrolysis constant

Rate Constants for Aquation and for Base Hydrolysis of Selected penta-ammine-Cobalt(III) Complexes a... 

The second-order rate constants k for the base hydrolysis of a number of cobalt(lll) complexes were measured with a simple flow apparatus using conductivity as a monitoring device. Equal concentrations (Ag) of reactants were used. Show that a plot of R,/R — R, vs time is linear, having slope s, and that... 

Table 6w4 Effect of Cobalt(lll) Coordination on Rate Constants for Base Hydrolysis of Amino Acid Amides...

Metal-ion catalysis has been extensively reviewed (Martell, 1968 Bender, 1971). It appears that metal ions will not affect ester hydrolysis reactions unless there is a second co-ordination site in the molecule in addition to the carbonyl group. Hence, hydrolysis of the usual types of esters is not catadysed by metal ions, but hydrolysis of amino-acid esters is subject to catalysis, presumably by polarization of the carbonyl group (KroU, 1952). Cobalt (II), copper (II), and manganese (II) ions promote hydrolysis of glycine ethyl ester at pH 7-3-7-9 and 25°, conditions under which it is otherwise quite stable (Kroll, 1952). The rate constants have maximum values when the ratio of metal ion to ester concentration is unity. Consequently, the most active species is a 1 1 complex. The rate constant increases with the ability of the metal ion to complex with 2unines. The scheme of equation (30) was postulated. The rate of hydrolysis of glycine ethyl... 

In the systems of Mo and Co sulfides, TAA was assumed to release sulfide ions by hydrolysis accelerated by hydrazine. Since the concentration of S2 in equilibrium with TAA is extremely low despite the exceedingly high release rate constant of S2- in the reversible reaction of Eq. (1), this assumption is reasonable if the concentrations of the free metal ions are too low for the nucleation of these metal sulfides. However, if the role of hydrazine is different than an accelerator of hydrolysis of TAA, and if the deposition rate of the metal sulfide monomers or the release rate of metal ions from the metal ion complexes such as orthomolybdate or cobalt... 

The kinetic effects of C02 in the base catalyzed hydrolysis of some carboxylato amine cobalt(III) complexes have been reported (80-82). In the base catalyzed hydrolysis of oxalatopentaammine-cobalt(III) (80), C02 retarded the reaction due to the formation of a virtually unreactive ion-pair, f (N H .) r, 2 2 COi ]. The equilibrium constant for formation of carbonate ion-pairs with (glycinato-O) (tetraethylene-pentamine)cobalt(III), (81) and (o-methoxybenzoato) (tetraethylenepentamine)cobalt(III) (82) were, however, much smaller than for the oxalatopentamminecobat(III) and a very weak rate retardation and virtually no effect was observed in the base catalyzed hydrolysis of the latter two complexes. 

A variety of N-O-chelated glycine amide and peptide complexes of the type [CoN4(GlyNR R2)]3+ have been prepared and their rates of base hydrolysis studied.169 The kinetics are consistent with Scheme 8. Attack of solvent hydroxide occurs at the carbonyl carbon of the chelated amide or peptide. Amide deprotonation gives an unreactive complex. Rate constants kOH are summarized in Table 16. Direct activation of the carbonyl group by cobalt(III) leads to rate accelerations of ca. 104-106-fold. More recent investigations160-161 have dealt with... 

Table 16 Rate Constants for the Base Hydrolysis of Ester, Amide and Peptide Bonds in Various Cobalt(III) Complexes (25 °C, / = 1.0 M)a...

For the reaction of MOH(n 1)+ with propionic anhydride,200 the Bronsted plot of log kMOH versus the pKa of MOH2n+ follows a smooth curve if the values for HzO and OH- are included (Figure 4). However, if the line is drawn to exclude the fcHj0 value, a Bronsted /3 of ca, 0.25 is obtained. Although kMOH for [Co(NH3)5OH]2+ (3 M s 1) is some 103-fold less than k0H, this reaction will compete favourably at neutral pH with base hydrolysis. At pH 7 where the cobalt(III) complex exists almost completely as the MOH2+ species the observed first order rate constant for nucleophilic attack by OH would be ca. 10-4 s 1. AIM solution of [Co(NH3)5OH]2+ would give a value of kobs 2.5 s 1, a rate acceleration of > 104-fold. Since the effective concentration of a nucleophile in the intramolecular reaction could be ca. 102 M, rate accelerations of 10° are possible. The role of the metal ion in such reactions is to provide an effective concentration of an efficient nucleophile at low pH. 

Figure 2, which summarizes values of the pseudo first-order rate constant for ADP hydrolysis at pH 7.0 and 20°C, illustrates the influences of the stoichiometric cobalt to ADP ratio, the total stoichiometric concentration, and preformation of the 1 1 complex at pH 4.0. The small values of kQ s observed for the 1 1... 

Complexes of cobalt (III) containing coordinated perrhenate ion have not previously been reported. % They are of special interest because of their close relationship to the unstable perchlorate complexes and are of use in studies on the mechanism of hydrolysis. Since the equilibrium constant of formation is small and [Co(NH )5(HjO)](Re04) -2H20 is relatively insoluble, the present procedure is similar to that used previously for mom common complex ions. 

The hydrolysis of adenosine 5 -triphosphate (ATP) in the presence of various cobalt(III) complexes has been studied. Complexes such as [Co(en)3] which have no available sites for coordination of the substrate display no catalytic activity. Complexes having one site or two sites in a trans configuration such as tetraethylenepentaminecobalt(III) or bis(dimethylgly-oximato)cobalt III) slightly enhance ATP hydrolysis. However, complexes with two available sites in a cis configuration such as cis-a- or crs-jS-Co(trien) exhibit considerable activity. Both the reactions ATP+HjO- ADP+Pj and ATP+H20- AMP+PPi occur with these systems. The complex [Co(dien)] effectively enhances the hydrolysis of ATP to ADP+Pj. At pH 4.0 the uncatalyzed hydrolysis rate constant for ATP hydrolysis is 1.18xl0 s at 50 °C. For ATP UxlO M) and [Co (dien)] "" (2xlO M) at pH 4.0, = 1.75X 10 at 50°C, a rate... 

The effectiveness of the binuclear complex 11 (Fig. 13), with two mononuclear cyclen-cobalt(III) units linked together by an anthra-cenyl spacer (cyclen = 1,4,7,10-tetraazacyclododecane), was compared with the monomer in the hydrolysis of phosphate monoesters (354). The reaction assisted by this rigid binuclear complex, having a phosphate-sized pocket, was 10 times faster than that promoted in the presence of two equivalents of the single cyclen-Co complex. In these experiments the substrate concentration was 25 pM and the total cobalt concentration was 2 mM at 25°C and neutral pH (354). No such cooperativity could be noted using a diester substrate because the pseudo-first-order rate constants were similar for both 11 and the mononuclear complex. With 11 as catalyst, an overall rate enhancement of 10 was achieved over the uncatalyzed hydrolysis of paranitrophenyl phosphate monoester as substrate. 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

The hydrolysis of ethylene phosphate in hydroxide ion solution proceeds with a rate constant of 5 x 10" L-mol" s" (100). The O-P-0 angle in the ring of ethylene phosphate of 99° is expected to be rather similar to that for the four-membered ring incorporating the cobalt and phosphorus centers. Therefore it is likely that reaction at the strained P center of the complex is eclipsed by a more rapid metal-ligand cleavage reaction. This problem can be circumvented by the use of metal ion complexes of Ir(III) where the metal-ligand bonds are more inert as the locus for the reaction. 

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