Mixed ligands formation constant

Van der Linden WE, Beers C (1975) Formation constants of mercury(II) with some buffer/ masking agents and the formation of mixed-ligand complexes. Talanta 22 89-92... 

Equation 1.8 suggests that the co-proportionation constant, calculated on a statistical basis, will always be above unity. Thus, the statistical redistribution of ligands will favor the formation of mixed-ligand complexes. 

Solution equilibria of Fe(III)/Cr(III) in methionine and Fe(III)/Cr(III) in methio-nine/penicillamine complex systems were studied by paper electrophoresis and were reported by Tewari [38]. The formation of 1 1 1 mixed ligand complexes is inferred, and the stability constants of the complexes at 35 °C and 0.1 M ionic strength were 6.80 0.09 (Fe(III) methionine penicillamine) and 4.60 0.16 (Cr(III)-methionine-penicillamine), respectively. 

During the early sixties Thompson and Loraas (77) reported the formation of mixed complexes of reasonable stability (log K 3.0—5.3) between lanthanide—HEDTA and ligands such as EDDA (N,N -ethylenediaminediacetic acid), HIMDA (N-hydroxyethyliminodiacetic acid) and IMDA (iminodiacetic acid). This observation together with the remarkably large formation constants (72) for the bis-EDDA complexes [log A2 =4.73 (La) 8.48 (Lu)] suggested a coordination number larger than six for the tripositive lanthanide ions in aqueous solution, in view of the fact that mixed chelates of the t5q>e M (HEDTA) (IMDA) axe not formed when M =Co(II), Ni(II) or Cd(II). 

Other arsenic-containing ligands reported include mixed donor species with S212 or N213,214 as the other donor atom. The ligand bis(carboxymethyl)(o-methylthiophenyl)arsine (24) contains S and As donors in a position suitable for chelation. Formation constants for its complexes with Ag1 are given in Table 30. The silver complexes of the m and p derivatives were insoluble under the conditions used and no data could be collected.212... 

The zinc(II) complex with methyl-5-nitrosobarbituric acid has been reported.571 as have formation constants for mixed ligand complexes of Zn11 and Cd11 involving cytosine572 and 2-aminopyrimidine573 respectively. 

Mercaptopyridine 1-oxide (46) acts as an S—O bidentate on deprotonation of the thiol group. Formation constants for the 1 1 chelates of this ligand with a variety of metal ions were compiled by Garvey et al.m in a review of the coordination chemistry of heterocyclic N-oxides. Synthetic, structural and bonding aspects of these mixed donor ligands were covered in later reviews by Karayannis et a/.166,167... 

An interesting correlation has been observed53 between the formation constant XCuL of the metal complex and its catalytic activity in a mixed ligand with an amino acid ester. Large values of XCUL (equation 13) lead to lower base hydrolysis rates in the ternary complex. The Lewis... 

Bivalent zinc, cadmium, nickel, and copper have been found to form ternary mixed-ligand complexes with histidine or edta and polyphenols.268 The formation constants for the ternary complexes are less than those for the binary systems. 

Reaction (4) is an equilibrium reaction and is generally slow at room temperature. The proposed reaction mechanism involves the formation of a stacking dinuclear intermediate (4) that isomerizes through the intermediate (5) to a mixed-ligand dinuclear species (6). Dissociation of 6 forming the mixed-ligand complexes is the rate-limiting step (Scheme 3). Consistent with the proposed mechanism, addition of a base such as a phosphine or an arsine, which is likely to suppress the dissociation of 6, inhibits the reaction. The equilibrium constant is not affected by the presence of an excess base (53). 

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

The properties of complex ions will be discussed in more detail in Chapter 20. For now we will just look at the equilibria involving these species. Metal ions add ligands one at a time in steps characterized by equilibrium constants called formation constants, or stability constants. For example, when solutions containing Ag+ ions and NH3 molecules are mixed, the following reactions take place ... 

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