Ligands chelate-forming

The first two terms of the right-hand side of the equation are sometimes combined and expressed as E which is called the standard oxidation potential for the chelate system. If the chelation is strong and the ligand is in excess, the metal would be almost entirely in the chelated forms, and [M L] and [M g L] would essentially be equal to the total concentrations of the oxidized and reduced forms of the metal. If, as is usual, the oxidized form is the more strongly chelated K > ), the oxidation potential of a system is increased by the addition of the chelant. 

If a neutral chelate formed from a ligand such as acetylacetone is sufficiently soluble in water not to precipitate, it may stiH be extracted into an immiscible solvent and thus separated from the other constituents of the water phase. Metal recovery processes (see Mineral recovery and processing), such as from dilute leach dump Hquors, and analytical procedures are based on this phase-transfer process, as with precipitation. Solvent extraction theory and many separation systems have been reviewed (42). 

Figure 15.2 (p. 412) shows the structure of the chelates formed by copper(II) with these ligands. Notice that in both of these complex ions, the coordination number of copper(II) is 4. The central cation is bonded to four atoms, two from each ligand. 

Charles s and Gay-Lussac s law Relation stating that at constant P and n, the volume of a gas is directly proportional to its absolute temperature, 106-107, 111 Chelating agent Complexing ligand that forms more than one bond with a central metal atom the complex formed is called a chelate, 411-412 natural, 424-425 synthetic, 424-425 Chemical equation Expression that describes the nature and relative amounts of reactants and products in a reaction, 60-61. See also Equation, net ionic. 

After 19 hours, no reaction between the zinc chelate 2 and benzaldehyde can be detected at 20 °C. However, 10 mol % of the zinc chelate effectively catalyzes theenantioselective addition of diethylzinc to aromatic aldehydes. The predominant formation of the S-configurated products, effected by this conformationally unambiguous catalyst, can be explained by a six-mem-bered cyclic transition state assembly17. The fact that the zinc chelate formed from ligand M is an equally effective catalyst clearly demonstrates that activation of the aldehyde moiety does not occur as a consequence of hydrogen bond formation between the ammonium proton of the pyrrolidine unit and the aldehydic oxygen. 

Complexes with coordination number 6 tend to be octahedral those with coordination number 4 are either tetrahedral or square planar. Polydentate ligands can form chelates. 

Charles s law The volume of a given sample of gas at constant pressure is directly proportional to its absolute temperature V T. chelate A complex containing at least one polydentate ligand that forms a ring of atoms including the central metal atom. Example [Co(en)3]3+. chemical analysis The determination of the chemical composition of a sample. See also qualitative, quantitative. 

Thiolates (RS ) represent an extensive family of ligands, and include chelating forms. Thiolates are known to act as monodentate donors, but often act in a bridging role. There is a clear biological interest, through participation of thiolates (cysteine residues) as donors in many metalloproteins both as terminal S donors and bridging ligands in, for example, Fe S clusters. 

Ligands with S donors in addition to N and or O donors bound to Co11 are reasonably large in number. For example, the 4-amino-3-alkyl-l,2,4-triazole-5-thione can bind Co11 as a chelate employing the primary amine and thione substituents on the five-membered ring,510 whereas the trifluoromethyl ligand (afmt) forms [Co(afmt)2(H20)2](N03)2, defined as the A -irons isomer... 

Dinuclear complexes with bridging phosphido, hydrido, and diphosphine ligands were formed via some interesting transformations, such as P—C bond formation, P—H bond activation, and conversion of a chelate diphosphine to one bridging two metal centers.259... 

Because of the chelate effect, ligands that can displace two or more water molecules from the coordination sphere of the metal generally form stable complexes. One ligand that forms very stable complexes is the anion ethylenediaminetetraacetate (EDTA4-),... 

MePO2- or PME2- (Table XIX), but the open closed equilibrium lies very much on the side of the chelated form of the complex (87% for the Ca2+ complex - compare 15% for [Ca(atp)]2 and just 7% for [Ca(amp)] (695)). The availability of stability constants both for methylphosphonate and for benzimidazole (a purine model) complexes means that the chelate effect for complexes of (1H-benzimidazol-2-yl-methyl)phosphonate can be discussed without the usual complications, such as the differences between ethane-1,2-diamine and two ammonia or two methylamine ligands and disparities between units (704). 

Scheme 4 Proposed reaction of Pd-H moiety reacting with excess ligand to form the inactive bis-chelated complex...

Pei et al. [412] reported an alternating fluorene copolymer 331 with 2,2 -bipyridyl in a side chain that emitted at 422 nm. Treating this polymer with Eu3+ chelates formed the polymeric complexes 332-334. Their emission was governed by intramolecular Forster energy transfer, whose efficiency depends on the structure of the ligands and the Eu3+ content (Scheme 2.49) [412], The most effective energy transfer manifested itself in a single red emission band at 612 nm for the complex 332 with a maximum intensity achieved at —25 mol% content of Eu3+. 

The scheme in Fig. 5.5 indicates that the ligand, for example, oxalate, is adsorbed very fast in comparison to the dissolution reaction thus, adsorption equilibrium may be assumed. The surface chelate formed is able to weaken the original Al-oxygen bonds on the surface of the crystal lattice. The detachment of the oxalato-aluminum species is the slow and rate-determining step the initial sites are completely regenerated subsequent to the detachment step provided that the concentrations of the reactants are kept constant, steady state conditions with regard to the oxide surface species are established (Table 5.1). If, furthermore, the system is far from dissolution equilibrium, the back reaction can be neglected, and constant dissolution rates occur. 

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