Square planar complexes rate constant

The second-order redox reaction, giving rise to the rate constant k2, is accompanied also by loss of the iron(II) complex by hydrolysis, which leads to the /tj term. The latter can be more accurately measured in the absence of Tl(III). The kinetics of substitution of many square-planar complexes conform to behavior (c), see Sec. 4.6. It is important to note that an intercept might be accurately defined and conclusive only if low concentrations of B are used. In the base catalyzed conversion... 

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

Figure 2. Bimolecular displacement mechanism for substitution reactions of square planar complexes. ka is the rate constant for the solvent path and ky is the rate constant for the direct reagent path.

The thermodynamics and kinetics of H+ binding to cobalt(I) and nickel(I) macrocycles have been determined. The pAia of Ni(cyclam)(H), / / 5 5 -NiHTIM(H) + and A-rac-CoHMD(H) + are 1.8, 1.9 and 11.7, respectively [14, 24, 27]. As seen from Table 3, protonation rate constants for A-rac-CoHMD depend on acid strength. The results are consistent with an associative reaction of the square-planar complex with an acid, HA. Whereas the spectrum of 7V-rac-CoHMD(H) + suggests the formation of a [Co (H )] + species with an absorption band at 440 nm (520 M cm ), Ni(cyclam)(H) + shows no significant absorbance in the 300-700 nm region [14, 24]. 

The Associative Mechanism The A mechanism can be expected to predominate in complexes that can expand their coordination number relatively easily. The obvious candidates are the square planar complexes of Pt11, Pdn, and Aum, for which many comparable five-coordinate complexes are also known. The salient feature of ligand substitution kinetics in these square planar complexes, however, is not the detectability of five-coordinate intermediates—the reaction rates are usually accurately second order overall—but rather the wide-ranging dependence of the second-order rate constants on the nature of the incoming ligand. This implies that in the scheme... 

Fig. 25.2 (a) Determination of the ki and /c2 rate constants (equation 25.11) from the observed rate data for ligand substitution in a square planar complex Y is the entering ligand. The dotted part of the line represents an extrapolation, (b) Plots of against concentration of the entering group for the reactions of /raw -[PtCl2(py)2] with [SCN] or with I both reactions were carried out in MeOH and so there is a common intercept. [Data from U. Belluco et al. (1965) J. Am. Chem. Soc., vol. 87, p. 241. 

Since substitution reactions of square-planar complexes in most cases tend to follow an associative mechanism, a large number of complexes were synthesized in efforts to determine whether a changeover in mechanism is induced. The increase in steric hindrance caused by introducing alkyl substituents on the three N donor atoms of diethylenetriamine (dien) in going from dien to Mesdien to Etsdien, caused a decrease in the aquation rate constant for [Pd(Rsdien)Cl]+ of six orders of magnitude, but not a changeover in mechanism. The reported activation... 

Substitution Reactions of Square-Planar Complexes 459 TABLE 12.13 Rate Constants for Leaving Groups... 

Table 4.8 Second order rate constants for similar reactions of square-planar complexes of nickel(II), palladium(ll) and platinum(II) at 25°C...

Determination of activation volumes from the pressure dependence of rate constants is a useful tool for the classification of mechanisms. A comprehensive review of a wide range of reactions with data in tabular form covering the period 1966—1976 has appeared. Kelm and co-workers have summarized results for co-ordination compounds, including square-planar complexes. " ... 

The rate behavior of associative ligand substitutions of square-planar complexes reflects the two pathways that often occur in parallel with and without solvent assistance. Thus, the corresponding rate law provided in Equation 5.8 contains two terms, one of which tjrpically corresponds to the rate constant for associative substitution with solvent assistance and one of which typically corresponds to the rate constant for associative substitution without solvent assistance. Often a plot of versus [Y] appears like the plot... 

This two-term form, normal for square-planar complexes, is extremely unusual for substitution at an octahedral complex. The tantalum(v) appears to be present in the reaction system solely as [TaF ], but of course [TaF ] is a stable anion so that parallel associative and dissociative paths for fluoride exchange represent a reasonable mechanism. Rate constants and activation parameters are listed in Table 9. The activation entropy for the ki term is entirely consistent with associative fluoride exchange via a... 

AuMejClCSbPhs), or AuMe2(SCN)(PPhs) is slow. Exchange of phenyl-dimethylphosphine with RhCl(CO)(PMe2Ph)2 was reported to be fast on the n.m.r. time-scale, but a reinterpretation of these n.m.r. observations suggests that this is not the case. Indeed, it seems that the rate constant for a bimolecular exchange process is about 2.5 x 10 1 mol s which is more plausible for a substitution at this type of square-planar complex. The exchange of free PFs with planar rhodium(i)-PF3 complexes is, however, reported to be fast by n.m.r. standards. ... 

Solvent The identification of solvent path for the substitution reactions in square planar complexes has increased the importance of solvent effect on the rate. The solvents with greater coordinating abfiity show faster rates of substitution reactions. The observed rate constants for some of the popular solvents vary as ROH < H2O < CH3NO2 < (CH3)2S0... 

Rate constants for the substitution reactions of square-planar dithio-phosphates and dithiocarbonate complexes of Ni(II), Pd(II), and Pt(II), with ethylenediamine and cyanide ion as nucleophiles, have been measured in methanol. The results were compared with those obtained in previous investigations, and interpreted in terms of the stabilities of 5-coordinate species that are formed prior to substitution (377). 

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

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