Polydentate ligands chelating

To explore the coordination abilities of mixed phenol-bpy/phen/terpy polydentate chelating ligands, Ward et al. investigated complexes (319) (Cu-Cu 3.050 A moderate AF coupling),281,282 (320) (moderate AF coupling),283 and (321) (Cu-Cu 3.324 A weak AF coupling).28... 

Eu(Tc)(cit) is 1 1 2 in this case. The fluorescence intensity of the 615-nm emission line of [Eu(Tc)(cit)2 ] is 22 times stronger than that of [Eu(Tc)]. Citrate, as a polydentate ligand, can chelate the Eu3+ ion via the oxygen atoms of its carboxy and hydroxy groups. It is assumed that citrate displaces water molecules which ligate to the 8- and/or 9-coordination sites of the Eu3+ ion and quench its fluorescence. Table 5 summarizes the luminescence properties of the [Eu(Tc)] complex (1 1 stoichiometry) and the corresponding chelate complexes with the intermediates of the citrate cycle. Citrate concentrations can be imaged with this luminescent probe by means of the RLI method (Fig. 15) [107]. 

Per Section 1-10.6.2.2. of [48] "In the nomenclature of polydentate chelate complexes, single ligating atom attachments of a polyatomic ligand to a coordinating centre are indicated by the italic element symbol preceded by the Greek letter kappa, k."... 

Monodentate ligand Bidentate, chelating ligand Polydentate, macrocyclic ligand Encapsulating, macropolycyclic ligand... 

Not only does chelation make the complex more stable, but it also forces the donor atoms to take up adjacent or cis sites in the resulting complex. Equation (2) shows how displacement of a chelating carbonate ion gives the unusual cis dichloride product instead of the thermodynamically more stable trans dichloride. Polydentate chelating ligands with three or more donor atoms also exist. Macrocyclic ligands, such as (4) and (5), confer an additional increment in the formation constant (the macrocyclic effect) they have been given trivial names, such as cryptates (4) and sepulchrates (5).i... 

A main property of humus acids, which attracts attention to them, is their capability of cation exchange. It is associated with the large size of their molecules that allows to consider them as suspended particles with variable number of active centers. In acidic water free forms of humic and fulvic acids are possible. With the increase in pH they lose hydrogen ions and convert in polydentate (chelate) ligands. That facilitates attaching of cations and the formation of salts called humates and fulvates, or more complex salts. 

We have seen how the pH and the presence of several complexing agents can be taken into account in equilibrium calculations. If, as happens often, we can identify one reaction in the array of reactions as the principal reaction, then all the others can be properly be called side reactions, and treated in a convenient manner. For example, in complexometric titrations of metal ions with EDTA or some other polydentate chelating titrant, the presence of auxiliary ligands like NHj, citrate anion, etc., can best be accounted for by the use of the conditional constant, first introduced by Schwarzenbach and widely applied by Ringbom. 

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