Binuclear intermediates

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

It will be noted that conversion of the intermediate V(OH) Cr " to products involves a different number of H ions than its conversion back to reactants. It is considered likely that the binuclear intermediate has an inner-sphere structure. On applying the steady-state approximation to the concentration of this intermediate, it follows that... 

To be consistent with the observed first-order dependences on Pu(VI) and U(IV), it is necessary that steps (13.24) and (13.25) do not occur simultaneously. The rate of reaction decreases with increasing hydrogen-ion concentration. It appears that two activated complexes (and a binuclear intermediate) are involved in the... 

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

It is supposed that binuclear intermediates involving M and M, occur in the various paths. The most studied systems involve edta complexes. 

The outer-sphere pathway k ) produces the final products directly, as shown by a rapid increase in absorbance at 474 nm, which is a maximum for Ru(NH3)5pz. At the same time, a rapid inner-sphere (A ,) production of the binuclear complex takes place. A slower absorbance increase at 474 nm arises from the back electron transfer in the binuclear intermediate This produces the original reactants which then undergo outer-sphere reaction (Atj). 

Implicit in the foregoing is the assumption that the oxidation states can be assigned with certainty in the binuclear intermediate, and this issue will now be considered explicitly. In the present case, a strong item of evidence is that the n <- %d absorption, characteristic of the Ru11—heterocycle, is observed in the successor complex an Rum-heterocycle shows no absorption in the same region of the spectrum. The d—d absorption characteristic of Crm in an oxygen environment is also observed. Finally, the rate of aquation at the chromium center is characteristic of the Crm state. 

Owing to the slowness of the reactions with Pt11, displacement of the N(l) proton in inosine and guanosine and their nucleotides by Pt11 is diverted by prior coordination and kinetic fixation at N(7). There is often insufficient Pt11 to form significant amounts of the binuclear intermediate, and hence no convenient pathway to generate N(l)-metallated species. 

An inner-sphere intramolecular electron transfer has been observed also in the reaction of chromium(II) salts with [Co(A-acacCN)(NH3)5] + °. This reaction proceeds via the binuclear intermediate [(NH3)5Co(A-acacCN)Cr]" +, which evolves to [Co(NH3)5(H20)] and [Cr(0,0 -acacCN)(H20)4] + ° . The mechanism is supported by the observation that addition of non-reducing metal ions such as Zn(II), Ni(II) or Ba(II) to the reaction mixture causes a decrease of the rate constant. ... 

Reductions of U(VI). Cr(II) was found to react rapidly with U(VI) to give a green binuclear intermediate (55) which is probably analogous to the substance which forms when Cr(II) and Np(VI) or Cr(IH) and Np(V) are mixed (89, 90). The intermediate decomposes to U(VI), U(IV), and Cr(HI) with an apparent half-time at 0°C. ranging from 4 to 8 min. Gordon (18) has shown that transfer of oxygen from to Gr(H20)6 is efficient in this reaction. 

Hindman et al. (28, 29, 30) have investigated the effect of D2O on the Np(IV)-Np(VI) reaction and redetermined the acid dependence. The previously determined dependence in H2O (30) could be interpreted in terms of either consecutive or parallel rate-determining reactions (51). We have now used the newer data and a least-squares procedure to compare the two mechanisms. In H2O, consecutive reactions fit the data better than parallel reactions the root-mean-square deviations are 3.63 and 3.81% respectively. In D2O the corresponding deviations are 7.79% and 4.18%. It is concluded that, unlike the analogous U(IV)-Pu( VI) reaction (51), there is no strong evidence for consecutive reactions and a binuclear intermediate. This reaction has been reinvestigated by Rykov and Yakovlev (76), who report higher rate constants under comparable conditions. 

The net reaction for the reduction of Pu(VI) to Pu(V) by Fe(II) is quite simple in spite of this a complicated three-term hydrogen ion dependence was found (56). A mechanism which involves both outer-sphere and inner-sphere activated complexes is favored. The inner-sphere complexes are supported by evidence for consecutive reactions and a binuclear intermediate. 

The detection of binuclear intermediates is important since it provides evidence for inner-sphere mechanisms conversely, outer-sphere mechanisms preclude their formation. It is significant that formally identical reaction sets do not necessarily all display binuclear intermediates. Not only might this be caused by a lack of the proper kinetic requirements for detectability but also by a lack of occurrence. This can be rationalized by pointing out that it is not necessary for these reaction sets to occur all by inner-sphere or all by outer-sphere mechanisms. In fact, examples of single reactions which apparently occur simultaneously by both mechanisms are Np( VI)-Cr(II) (89, 90), Pu( VI)-Fe(II) (56), and V(IV)-V(II) (57). 

Afft over-all would necessarily exceed AH for process (16), and 4 kcal appears to be a conservative lower limit for AH, even taking into consideration the probable tetragonal symmetry of Cr(OH2)8 . The high cost in energy for substitution can be avoided if we formulate the substitution process giving rise to the binuclear intermediate species as an... 

The study of binuclear intermediates is strongly called for. Thus far such species have not been detected in a direct measurement, but it seems likely that with properly chosen cations, ligands, and conditions, appreciable concentrations can be built up. This subject has been a matter of concern in investigations of interaction absorption, but in most of the... 

It has been postulated that the formation of a /r-Tj iTj -vinyl ligand in binuclear intermediates involves the activation of a coordinated olefin by one metal center followed by an intramolecular oxidative addition of a... 

Tetracarbonylalkyl derivatives of cobalt(I) have low stability. As early as 1964 it had been noted that ketones are formed in the thermal decomposition of CoR(CO)4 (R = Me, Et) presumably involving a binuclear intermediate or an intermolecular mechanism. The mechanism of acetone formation was studied for other cobalt systems that are more easily handled, namely, Co(>/ -C5H5)Me2(PMe3) and Co2(ti -CsH )2Me2(fi2-CO)2 . Upon carbonylation, in the former case, the transient carbonyl derivative Co()j -C5H5)Me2(CO) was observed spectroscopically, whereupon it underwent carbon monoxide insertion to give an acetyl-methyl complex, followed by reductive elimination of acetone ... 

In electron transfer between metal ions, a metal-ion catalyst normally reacts by nonassociative activation, in which the species do not form long-lived binuclear intermediates. The catalytic process often can be rationalized by reactivity patterns e.g., the Cu catalyses of the oxidation "of V(III) by Fe(III). This catalysis by Cu occurs by outer-sphere mechanisms as in ... 

---