Cations and ligands

Formation constant (Kf) Equilibrium constant for the formation of a complex ion from the corresponding cation and ligands, 422-425,639 Formic acid, 595... 

C18-0085. Write the chemical formulas, including charge, of the complexes that form between the following metal cations and ligands (a) Fe " and CN", coordination number = 6 (b) and NH3, coordination number = 4 and (c) and ethylenediamine (en), coordination number = 6. 

These processes include the thermal decomposition of all polynuclear clusters and also binuclear clusters with organic cations and ligands (Table 3). As a rule,... 

Figs. 2.8 and 2.9 exemplify the typical pH dependence for cation and ligand adsorption. As Fig. 2.9 illustrates the adsorption of the ligand A (or HA) goes through a maximum at a pH value that is near the pK value of HA. All kind of explanations have been given for the pH-dependence of this maximum it is important to realize that this maximum is a consequence of the mass law. 

It is possible to write the formation of the 2 1 complex directly from the cation and ligand as follows ... 

Fig. 6.5. The Gibbs energy for the formation of a hydrated cation from a gaseous cation (AGi) and for the formation of three complexes (A( 2, curves 1,2,3) from gaseous cation and ligands of various types. Curve 1 a complex with the hydrated cation will not be formed ...

Jhe electronic interaction between cations and ligands is well known to affect the... 

The guest cations hitherto examined cover broadly uni- to trivalent and inorganic to organic ions that include alkali, alkaline earth, heavy and transition metal ions, as well as (ar)alkyl ammonium and diazonium ions. As to the complex stoichiometry between cation and ligand, both 1 1 stoichiometric and 1 2 sandwich complexes are analyzed. The solvent systems employed also vary widely from protic and aprotic homogeneous phase to binary-phase solvent extraction. 

Figure 15. Schematic drawing of complexation-induced desolvation from cation and ligand (solvation to the latter is not shown) the complex formation itself reduces the entropy of the system to some extent but the accompanying desolvation leads to much larger increase in entropy ascribed to the liberation of solvent molecules.

Solvation of cation and ligand. The solvation free energy increases in the order K+ < Na+ < Ca2+, hence less energy is required to (partially) desolvate K+ in order to bind it. 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

Equations can also be written for the addition of the third and fourth cyanide, with constants K3 and K4. In addition to the stepwise formation equilibria, we can write a single overall equation for the formation of a complex containing several ligands from the free cation and ligands (actually a summation of the other chemical equilibria). Further, Ks = K1K2K3K4. 

Depending on total concentrations, a dramatic change in the ionic strength results from the formation of a neutral complex from a metal cation and ligand anions. 

Cui XJ, Khlobystov AN, Chen XY et al (2009) Dynamic equilibria in solvent-mediated anion, cation and ligand exchange in transition-metal coordination polymers solid-state or recrystallization Chem Eur J 15 8861-8873... 

Gordy and Thomas (25), Allred (26), Wells (27), and Rosier and Lange (28). Unfortunately, such data are mostly lacking for polyatomic cations and ligands, although EN may be estimated or computed for such species from mineral solubility products (29l). ... 

Because bonding is in large part electrostatic, when a single ligand is considered, K ssoc values are usually proportional to z+/r+ or to z+z /(r. + r ) where the + and - subscripts denote cation and ligand charge and radii respectively (30). As z+/r+ (Ip) values increase, however, covalent bonding becomes important. For example, important covalency and cation deformation occurs when complexes are formed with Be " or species such as Fe, Al, and (6). 

Further, the dielectric constant of water associated with a complex is known to decrease as cation and ligand more closely approach each other (19, 56). Thus, Choppin and Unrein (57) suggest "effective" e values of 57.0 for MF+2 and 40.8 for MF+3 complexes. The drop in both d and e should increase the stability of multivalent cation complexes over monovalent ones. That the Fuoss equation roughly predicts AG° for 3+ and 4+ cation complexes although ignoring real changes in d and e, must therefore be considered fortuitous. 

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