Exchange Unidentate

Langford, R. Scharfe, and R. Jackson, Inorg. Nuclear Chem. Letters 1973, 9, 1033. 

The kinetics of cyanide exchange at [Fe(CN)5(py)] have been studied, in the presence and in the absence of added pyridine, by tracer techniques. 

The proposed mechanism here, as in similar substitution reactions of [Fe(CN)5(L)] complexes (c/. Sections 4 and 5 of this chapter), is 5 Nl(Hm). The transient [Fe(CN 5] - intermediate seems to contain two types of cyanide ligand of different labilities it does not seem possible to distinguish between the two possibilities of four labile and one inert cyanide or three labile and two inert cyanides. Cyanide ion seems much more labile in [Fe(CN)5(py)] than in [Fe(CN)5(NO)] or in [Fe(CN)e] . This high reactivity is explained by facile loss of cyanide from the [Fe(CN)5] intermediate, itself more easily generated from [Fe(CN)5(py)] than from the other two complexes.  

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The exchange proreeds in three steps. The first step is substitution of the free ligand at one end of the chelate to give the intermediate I(fca). The second is intramolecular proton transfer between the unidentate ligand in l(kb). The third is the reverse of the first (k a). Consequently, application of the steady-state approximation to the intermediate 1, whose concentration is reasonably assumed to be very low, provides... 

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

In some cases, the unidentate ligand is liberated at the end of the reaction. Usually, however, the ligand is found in both the reactant and the product. The effect has been most systematically examined for Ni(II). Coordinated NHj and polyamines have the largest accelerating influence. The rate acceleration induced by macrocycles resides primarily in reduced AFTI values (by 15-26 kJ mol ). The 6- and 5-coordination of solvated tetramethylcyclam complexes is controlled by the conformation at the 4 N-centers, 2 and 3. These complexes exchange by and 4 mechanisms, respectively, as indicated by positive and negative values (Table 4.9). Also Sec. 4.9. 

Attempts have been made to treat selective ion solvation in mixed solvents as ligand exchange reactions [36 e]. We express ion X (cation or anion) existing in solvent A by XA and in solvent B by XB,-. Here, nf=ri if the molecular size of A is very different from that of B or if A is unidentate and B is bidentate (e.g. PC and DME in the solvation of Li+). Otherwise, it is usual that n=n. Then, the ligand exchange reaction in the mixture of A and B will proceed as follows with the increase in the concentration of B ... 

Unidentate carbonate species are formed on heating bivalent cation-exchanged Y zeolites in C02 (286). The Ca2+ ions are involved in the formation of this surface carbonate (281) in CaY zeolites. Jacobs et at. (281) have shown that lattice oxygen must be incorporated in C02 to form the carbonate on a dehydrated zeolite. Provided there are some residual water molecules retained in the zeolite, carbonate formation is explained by the following reactions ... 

Among five types of species formed by CO2 adsorption on MgO, dynamic behavior of unidentate-type carbonate (species-1) upon desorption was investigated. Surface diffusion, O-exchange, and molecular substitution appreciably take place prior its desorption even below the room temperature. Since these dynamic processes necessitate bond rupture in species, MgO may be useful in CO2 conversion at low temperatures. 

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