Cobalt complexes rate constants

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... 

Measurements of electron transfer rates by pulse radiolysis studies of Co -P -V show a diffusion-controlled reduction of the viologen chromo-phore, followed by exergonic (AG = -0.80 eV) electron transfer to the cobalt, with rate constants of 630 and 360 s for the 16-mer and 20-mer complexes, respectively (Figure 5). The reaction proceeds according to Scheme II. The... 

Table 2 First-order rate constants and activation parameters for aquation of cobalt(jn)-dioxime-halide and -pseudohalide complexes rate constants at 50 °C in aqueous solution...

Results of relaxation measurements on spin-state equilibria in solution are available for complexes of iron(II), iron(III), and cobalt(II). The results comprise values of relaxation time r, rate constants for the forward and reverse reactions feLH activation parameters AH and AS for the two opposed... 

The most significant results obtained for complexes of iron(II) are collected in Table 3. The data derive from laser Raman temperature-jump measurements, ultrasonic relaxation, and the application of the photoperturbation technique. Where the results of two or three methods are available, a gratifying agreement is found. The rate constants span the narrow range between 4 x 10 and 2 X 10 s which shows that the spin-state interconversion process for iron(II) complexes is less rapid than for complexes of iron(III) and cobalt(II). 

An unusually slow relaxation has been observed for the 2,6-pyridine-dicarboxaldimine cobalt(II) complex [Co(2,6-(CH3NH=CH)2py)2](PFg)2 in solution. Thus a relaxation time -c = 83 ns has been reported [99], the rate constants being among the lowest found. It has been suggested that nonelectronic factors such as partial ligand dissociation, steric effects or solvent interaction may be rate determining in this equilibrium. 

Cobalt(II) Complexes. Spin conversion rates greater than 10 s have been estimated for [Co(terpy)2](PFg)2 [29]. Although Ar 0.04 A is available for the [Co(terpy)2] complex [132] there is a large uncertainty concerning the magnitude of electronic coupling. Therefore, no attempt has been made to calculate the rate constant. 

Electrocatalysis employing Co complexes as catalysts may have the complex in solution, adsorbed onto the electrode surface, or covalently bound to the electrode surface. This is exemplified with some selected examples. Cobalt(I) coordinatively unsaturated complexes of 2,2 -dipyridine promote the electrochemical oxidation of organic halides, the apparent rate constant showing a first order dependence on substrate concentration.1398,1399 Catalytic reduction of dioxygen has been observed on a glassy carbon electrode to which a cobalt(III) macrocycle tetraamine complex has been adsorbed.1400,1401... 

The first term on the right-hand side denotes the rate of dioxygen reaction with styrene (see Chapter 4) and the second term is the rate of catalytic free radical generation via the reaction of styrene with dioxygen catalyzed by cobaltous stearate or cobaltous acetylacetonate. The rate constants were found to be ki = 7.45 x 10-6 L mol-1 s-1, k2 = 6.30 x 10 2 L2 mol 2 s 1 (cobaltous acetylacetonate), and k2 = 0.31L2 mol-2 s 2 (cobaltous stearate) (T = 388 K, solvent = PhCl [169]). The mechanism with intermediate complex formation was proposed. 

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

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

The above value of k4 1 s for bpy loss from Rh(bpy)3 + may be compared with k4 - 3 s for bpy loss from the formally related Co(bpy)32+ (13,14) Recently obtained results indicate that the rate constant for addition of bpy to Rh(bpy)2(H2O)2 (k 4 s 0.2 x lO Ms"1) is greater than that for the comparable cobalt(II) reaction (13,14) The more-or-less comparable labilities of Rh(bpy)3 T and Co(bpy)3 + are not unexpected in light of data for rates of ammonia loss from the two metal centers which are also available ammonia loss from rhodium(II) is quite rapid (10 s 1 to 10 s l with loss from Rh(NH3)5 H20 + being much faster than from Rh(NH3)4 +, etc ) W t>ut somewhat slower than the comparable process for cobalt(II) (15) Of course, here the relative affinities of the two metals for NH3 are not known and so cannot be taken into account A further reason these comparisons lack great validity is that, although these Co(II) complexes contain 3d metal centers, Co(bpy)3 + and Co(NH3)n + are high-spin complexes i.e. the ground states are (t2g) (eg) whereas 4d species are expected to be low spin, (t2g) (eg)1. Furthermore, as will be seen shortly it is not clear that even "low spin 4d " is an adequate description of the... 

The plot of log kf vs log X, is linear over a wide range of rate constants (Fig. 2.7). Obviously, the faster the aquation, the more the reaction goes to completion The sloped is 1.0 and this indicates that the activated complex and the products closely resemble one another, that is, that has substantially separated from the cobalt and that therefore the... 

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... 

Besides Scheme 3.45, one more case of ferrocenylammoninm oxidation deserves to be considered. That is, the chemical oxidation of the confined species. fV-(ferrocenylmethylene)-A/,A/,Af-trimethylam-monium forms a remarkably stable inclnsion complex with cucurbituril (Jeon et al. 2005). Yuan and Macartney (2007) used aqueous solution of the bis(2,6-pyridinedicarboxylato)cobaltate(III) ion for comparative oxidation of free and included compounds. This oxidant does not bind to curcubituril. As it turned out, the inclusion significantly reduces the rate constants for the ferrocenyl-ferroceniumly transition. One of the important causes of the retardation observed is the steric hindrance due to close approach of the oxidant to the encapsulated ferrocene (Yuan and Macartney 2007). 

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... 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

Table 1 lists some of the binding constants and rate constants measured for the reaction of CO2 with redox-active molecules. Various techniques have been used to measure these constants including cyclic voltammetry, pulsed radiolysis, and bulk electrolysis followed by UV-visible spectral measurements. The binding constants span an enormous range from less than 1 to 10 M [13-17]. Co(I) and Ni(I) macrocyclic complexes have been studied in some detail [13-16]. For the cobalt complexes, the CO2 binding constants K) and second-order rate constants for CO2 binding (kf) are largely determined by the Co(II/I) reduction potentials... 

Our recent work shows that steric effects are important in electron transfer via bridging groups. For example, in the reduction of the carboxylatopentaammine-cobalt(III) complexes by Cr(II), the rate constants decrease as follows ... 

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... 

These equations are in line with Eq. (30), such that kx denotes the ring-opening rate constant of the protonated carbonato complex and / 2 is the decarboxylation rate constant of the ring-opened bicarbonato complex. Values of these rate constants and the acid dissociation constants of some protonated carbonato complexes of cobalt(TII) (see Table III) reflect the ligand dependence with respect to charge variations, steric constraint, and donor properties of the non-labile ligands. 

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