Nucleophilic attack complex rate laws

The constant s, characteristic of the substrate complex, reflects its sensitivity to variation in nucleophilicity as assessed by the Ptpy2Cl2 reaction. It is called the nucleophilic discrimination factor (ndf). The intercept log k turns out to be related to the value of the k term in the rate law for the solvent in question. Some representative ligands involved in attack on Pt(II) complexes may be listed in order of decreasing as follows ... 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

All these features have been initially interpreted102-104 in terms of a molecular mechanism involving two successive alkene-iodine complexes of 1 1 and 1 2 stochiometries the second of which evolves by internal nucleophilic attack of the uncomplexed double bond to the diiodo derivative (equation 87). The intramolecular attack of the second double bond has been regarded as rate determining, owing to the fact that the overall rate law is second order in iodine rather than the usual third order. Nevertheless more... 

Nitrosyl complexes, in which Vno > 1886 cm or Fnq > 13.8 mdyn/A, usually react as electrophilic nitrosating agents so that the ligand can be considered NO [26]. Nucleophilic attack on the nitrosyl nitrogen is a common reaction encountered in the chemistry of nitroprusside and the rates and activation parameters for a number of different nucleophiles are listed in Table 3. Hydr-oxylamine adducts to nitroprusside via a rate law that is first order in the complex, the ligand and hydroxide (k = 4.5 x 10 M s ). 

This rate law is of the same form as that seen for substitutions on other metal-carbonyl complexes, but the relative values of the two terms are very different from those observed in other systems. The ligand-dependent term predominates for ligands that are reasonably good nucleophiles, such as CNR, PBus, PPhs, and P(OPh)3. The relative values of k2 show a strong dependence on the nucleophilicity of the entering ligand, indicating nucleophihc attack on the metal complex. 

The mechanism of attack of the nucleophile (H20) on the coordinated alkene was unclear for many years. Does the nucleophile attack externally in a manner trans to Pd to give the hydroxyalkylpalladium complex (anti attack, path b, Scheme 9.5) or does intramolecular 1,2-insertion of the alkene between the metal and a coordinated OH group (formed after deprotonation) occur with to OH cis to Pd (syn attack, path b, Scheme 9.5) The observed rate law could be consistent with... 

Region II (pH 10 to 12.5 for (CO)5Cr=C(SMe)Ph, pH 10 to 11.5 for (CO)5W=C(SMe)Ph)). Nucleophilic attack by water is rate limiting. Just as is the case for the hydrolysis of alkoxy carbene complexes, there is also general base catalysis of water attack, hence feobsd is given by equation (79) ( f [B] term not shown in Schemes 11 and 12). This rate law implies that the relationships of equations (80) and (81) hold the fe2 H+ is again negligible and omitted from equation (81). 

The reaction between the Ni(I) complex of 12 and CH3I also has an SN2-type mechanism. The Ni(I) complex of 12 reacts with CH3I in a 2 1 stoichiometry in MeCN to yield the corresponding Ni(II) complexes, I, CH4, and CaHg. The rate law measured in MeCN is -d[Ni ]/d = A[Ni ][CH3l], The second-order rate constants increase in the order methyl > ethyl > propyl. The rate-determining step proposed is an SN2-type nucleophilic attack at R—X by Ni(I) complex to generate Ni -alkyl transients 149,150). 

Breslow et al. 19) have investigated the mechanism of the divalent metal ion catalyzed hydration of 2-cyano-l,10-phenanthroline to the corresponding amide [see Eq. (7) below]. The cupric ion catalyzed reaction is extremely rapid lyz < 10 sec at 25°, pH 6—7). The Ni(II) and Zn(II) reactions, although slower, are greatly accelerated [the Ni(II) reaction by a factor of 10 ] in comparison to the rate of hydration observed in the absence of divalent metal ion catalysis. The reaction obeys a rate law which is second-order over all first-order with respect to hydroxide ion concentration, and first-order with respect to the 1 1 metal ion-substrate complex concentration. These authors suggest that the metal ion acts as a Lewis (general) add in activating the nitrile for external nucleophilic attack by hydroxide ion, as illustrated in Eq. (7) ... 

The reversibility of the zwitter ion adduct formation in Eq. 8.6 also affected the rate law of the formation of amino-substituted alkylmetal complexes. Thus, kinetic studies indicated [33] that the rate of the formation of /i-aminoalkyl complex 4 in Scheme 8.20 was second-order with respect to the concentration of the amine, namely rate = [amine] [complex]. This is consistent with a reaction sequence shown in Scheme 8.20 involving a reversible formation of the zwitter ionic intermediate, followed by the rate-determining deprotonation by the second amine molecule. The observed rate constant appeared to contain contributions from both the equilibrium constant of the first step and the rate constant of the second deprotonation, so that the direct comparison of the rate of the initial nucleophilic attack at the coordinated alkene between Pd and Pt complexes was not possible. However, the higher overall reactivity (ca. 70 times) of Pd complex than Pt complex was consistent with the higher ionization potential of Pd than Pt. This difference in the ionization potential then would lead to the weaker jt basicity of Pd(II) than Pt(II) for jt back-donation to alkene jt orbital, and therefore facilitated the nucleophilic attack at the Pd-alkene complex more than that at the Pt complex. 

Electrophilic additions of Brs" to alkenes and alkynes have been carried out [48-50] both in [BMIM][Br] and in other ionic liquids bearing non-nucleophilic anions (Scheme 5.1-16). The reaction is always completely anti-stereospecific, independent of alkene or alkyne structure. It follows a second-order rate law, suggesting a concerted mechanism of the type reported for Brs" addition in aprotic molecular solvents, involving a product- and rate-determining nucleophilic attack by bromide on the alkene or alkyne-Br2 jt-complex initially formed. 

Note that this latter mechanism requires nucleophilic attack on a coordinated olefin which is more electrophilic owing to the charge on the complex. Thus, the rate laws derived from mechanisms incorporating (15) or (16) differ in the order of solvent, which illustrates the potential pitfalls and... 

Radiochromatographic techniques have been used to determine the rates of oxidation of cysteine by pertechnate ion, Tc04. The technetium(vu) is reduced by the thiol (and cysteine ethyl ester) to form a Tc complex which involves both S- and 7V-co-ordination of the amino-acid. The rate law is first order with respect to both [Tc ] and [RSH]. A hydrogen-ion dependence observed is attributed to the formation of pertechnic acid, the rate-determining step being the nucleophilic attack by the thiol at the metal centre of HTCO4. The oxidation of RSH (R=Et, Pr, or Bu) has been studied over the range 20—40 °C in aqueous alkaline solutions in the presence of metal phthalo-cyanines. The reaction is zero order with respect to [thiol], first order in phthalocyanin and decreases in the order M = Co>Mn> V>Feii. No effects are observed from the nature of the alkali cation. 

Similarly, in the reactions of /rani-[IrCl(CO)(PPh3)2] with aryl iodides, the rate constant for disappearance of Ir(I) species obeys the two term rate-law shown in (26). The second term, exhibiting an inverse dependence on the concentration of PPh3, is attributed to a highly reactive, 14-electron species [lrCl(CO)(PPh3)] (Mureinik et al., 1979). In (26), Kis the equilibrium constant for dissociation of PPh3 from the complex, 2 s the rate constant for nucleophilic attack of [IrCl(CO)(PPh3)] on Arl and refers to the solvent mediated pathway. 

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