Complex Formation with Unidentate Ligands

Several studies involving the formation and dissociation of monocomplexes of iron(iii) have been reported. Rowley and Sutin have followed the reaction  

Cavasino and Di Dio have studied the formation of the monocomplexes of iron(iii) with some substituted phenols (HA). The only reaction observed involves the hydrolysed metal species  

The reactions of Fe with acetic, propionic, and chloroacetic acids also have normal rate constants.  

In spite of a suggestion by Gusenkov that in two- and three-stage mechanisms the maxima observed in the ultrasonic dispersion curves of the reaction mixture may not agree with the eigenvalues from the characteristic equation (both as regards their number and the frequency of the external perturbation), the results of further work with this technique have been reported. Hemmes and Petrucci have reinvestigated the 

The formation of the complexes between the m-benzenedisulphonate ion and Mg +, Mn +, Co, Ni +, Cu +, and Zn + in anhydrous methanol has been investigated by the pressure-jump technique.  

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FIG. 6. Plot of log Kx values for Ag+ complexes with unidentate ligands vs log Kx values for corresponding complexes with CH3Hg+. Data at 25°C and ionic strength zero in aqueous solution, from Ref. (11). Ligands are classified as soft ( ) intermediate (O) hard, N-donors (O) hard, O-donors ( ), or F ( ). Formation constant data from Ref. (11). 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

Complex Formation Involving Unsubstituted Metal Ions with Unidentate Ligands and Solvent Exchange... 

Figure 12 [115] shows a series of complex formation titration curves, each of which represents a metal ion-ligand reaction that has an overall equilibrium constant of 1020. Curve A is associated with a reaction in which Mz+ with a coordination number of 4 reacts with a tetradentate ligand to form an ML type complex. Curve B relates to a reaction in which Mz+ reacts with bidentate ligands in two steps, first to give ML complexes, and finally close to 100% ML2 complexes in the final stages of the titration. The formation constant for the first step is 1012, and for the second 108. Curve C refers to a unidentate ligand that forms a series of complexes, ML, ML2. .. as the titration proceeds, until ultimately virtually 100% of Mz+ is in the ML4 complex form. The successive formation constants are 108 for ML, 106 for ML2, 104 for ML3, and 102 for ML4 complexes. 

The equilibrium constant for the formation of a metal complex is known as its stability constant. (Some authors, however, present the datum as its reciprocal, the instability constant of the complex, by analogy with the dissociation of a weak acid.) There are two kinds of stability constants stepwise (Ad, Ad, Ad,..., Kn) and overall (/ ). We will assume that there are six aqua ligands to be replaced by some other unidentate ligand Xx, in an aqueous solution of Mm+ ... 

Decarboxylation of carbonate complexes is usually effected by acid hydrolysis with the formation of a C02 free oxide or hydroxide complex.128 All such reactions involve a protonated (bicarbonate) intermediate but there are some useful deferences which, in many instances, may be reconciled with the three main structural types of carbonate complexes. Both unidentate and chelate carbonates readily yield C02 on acidification, while there is a greater resistance to C02 loss when the carbonate is a bridging ligand. Unidentate carbonate complexes decarboxylate with the initial formation of a bicarbonate intermediate and subsequent loss of C02 without rupture of the M—O bond, viz. structure (3). By contrast, in chelate carbonate complexes, cleavage of the M—O bond occurs (with ring opening) with the formation of a bicarbonate aqua ion before the loss of C02, viz. equation (5).29... 

The approximate statistical distribution of unidentate ligands on TV sites is a special case of the hypothesis of step-wise complex formation that all the intermediate complexes ML occur in mixtures with 0 < /Tpreparatively separate) and imperfect (labile, equilibrating in solution) complexes. The latter suggestion is similar to the influence of Aristotle s principle of the excluded middle on a housewife when she declares that a given compound either is toxic or not. Though most of the reactions of iridium(III) are slower than the reactions of carbon compounds in organic chemistry, there is little doubt... 

Figure 6.9. The chelate effect on complex formation of Cu aq with monodentate, bidentate, and tetradentate amines. pCu is plotted as a function of concentration in the left-hand diagram. On the right the relative degree of complexation as measured by ApCu as a function of concentration is depicted. The extent of complexing is larger with chelate complex formers than with monodentate ligands. Unidentate complexes are dissociated in dilute solutions while chelates remain essentially undissociated at great dilutions.

Complex formation by replacement of molecules in the solvated shell of a metal cation in aqueous solution with, for example, unidentate ligands,... 

The most widely used complexometric titration employing a unidentate ligand is the titration of cyanide with silver nitrate, a method introduced by Liebig in the 1850s. This method involves the formation of the soluble Ag(CN)J, as discussed in Feature 17-2. Other common inorganic complexing agents and their applications are listed in Table 17-1. 

N-3 Acyclic multidentate amine ligands yield polymeric solid complexes with Ag(I). Thus, diethylenetetramine and tris(2-aminoethyl)amine give 128 and 129, respectively. In the presence of unidentate ligands such as PPhs, PMe3 and /-BuNC polymer formation may be hampered, as in 130215. 

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