Unidentate Ligand Exchange

The pressure and temperature dependence for the substitution of bromide by iodide and thiourea in the sterically crowded complexes cis- and rra/w-[Pt(PEt3)-(2,4,6-Me3C6Ha)Br] have been determined by van Eldik, Palmer, and Kelm. For both the ki and k paths, the activation entropies are strongly negative and the activation volumes vary between —11 and -16 cm moLk This is consistent with an associative mechanism, contrary to previous suggestions.  

Kukushkin and O. V. Stefanova, Russ. J. Inorg. Chem. Engl. TransL), 1977,22,1844. 34 M. Cusumano, P. Marricchi, R. Romeo, V. Ricevuto, and U. Belluco, Inorg. Chim. Acta, 1979, 34, 169. 

Mureinik and Bidani have performed a study of the reaction of [Pt(terpy) ] with various nucleophiles with the aim of comparing the reactivity of the terpy and dien complexes. The rate constant for the solvent path is found to vary with the leaving ligand concentration according to l/(a[X]+Z ), a and b being constants. This is believed to indicate a deviation from the normal mechanism. The reason for this conclusion is not quite clear, since the experimental expression reported by the authors conforms to the first term of equation (2) if the experiments are performed at constant [Y] and varying excess of [X]. 

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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 ... 

Complex Formation Involving Unsubstituted Metal Ions Unidentate Ligand Substitution and Solvent Exchange... 

Lidiiim.— Ligand exchange between Me InL and HL, where L = [CFaCOCHCOR] with R = Me or Bu, obeys kinetics that are first-order in each reactant, and is sug ted to involve a species with two unidentate diketonate ligands, the slow step being rotation of one of these about a partial double bond. Kinetic data on the formation of a complex between indium(m) and murexide lead to a somewhat different stability constant to that obtained by other methods. The authors speculate on the possibility that more than one type of complex may be involved. Several further accounts of the formation and reactions of indium(i) species have appeared. ... 

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