Non-coordinating anion

As a weakly coordinating ions are referred to in the chemical ions that undergo only very weak interactions with other molecules or ions. These interactions are mainly related to the formation of coordination bonds. To emphasize that the ions are substantially independent from one another, and the terms free ion or naked ion are often used. While such ions have long been known in the gas phase and, increasingly, compounds are prepared having similar properties in solution or in the solid state.

For science are weakly coordinating ions of increasing importance, since they allow for the examination of highly reactive compounds having a variety of physical and chemical methods that can not be used in the gas phase.

Practical application find weakly coordinating ions, for example in the production of novel catalysts in the coordinative polymerization, in the development of ionic liquids as solvents for chemical reactions and electrochemistry.

  • 3.1 Covalently bound Gerüstanionen
  • 3.2 Stable Lewis acid - base complexes
  • 6.1 coordination
  • 6.2 lithium -ion batteries
  • 6.3 Ionic Liquids
  • 6.4 Organic Catalysis



A coordinative bond is generally understood as a chemical bond, wherein the bonding electrons are provided only by a binding partner. The best known of these compounds are ionic complexes. Here, more negatively charged anions are grouped around a positively charged cation. The anions to use a lone pair of electrons as a ligand to bind to the cation, the central atom. Here, the number of surrounding ions is the coordination number and the physical arrangement represented by the coordination polyhedron.

In solid state, ions are arranged in a lattice ion. Both cations and anions of a plurality of oppositely charged particles ( the counter ion ) is surrounded.

A "weak " Coordination in this context means that the binding energy of the coordinate bond is very low. As always, the anion contributes the bonding electrons, the coordination ability is mostly dependent on the nature of the anion. It is, however, (especially in solids and melts ) is also possible to influence the strength of the bond by the properties of the cation.

Free ions in the gas phase

Ions generated in the vacuum are due to the large spatial distances to all other atoms as free floating in space charge carriers. They are often in an ion source through targeted bombardment with electrons ( impact ionization ) or charge transfer by another ionized gas produced (chemical ionization) and mainly analyzed by mass spectrometry.

In the food industry, for example ionized air for pasteurizing beverages is used. Here, the high reactivity of the ions is used. This circumstance, however, that such ions usually have only a very short life and disintegrate almost immediately after their production or react further. Thus, it is not possible to carry out lengthy spectroscopy (NMR, IR, Raman, UV / VIS). The restriction to the gas phase and diffraction experiments such as X-ray diffraction or neutron scattering are impossible.

Free ions in solutions and solids

The definition of liquid and solid phases requires that particles always interact with each other. Therefore, there can be ions in these conditions is no " free".

Influence of the solvent

In solution, ions are surrounded and solvated by the solvent. The solvent acts as a dielectric ( insulator ) by inversely arranges to the charge of the ion and the electric field weakens as to the ion. The measure of this attenuation is the polarity of the solvent, which is expressed by its dielectric constant ( dK) as.

In a highly polar solvent such as water ( εr = 80) dissolved ions show little interaction with each other. However, the interactions with the solvent are stronger, which can be illustrated by the example of lithium-ion: Due to its large hydration shell of Li shows a much lower mobility than the much larger sodium or potassium ions.

The transition to non-polar solvents such as dichloromethane ( εr = 9) or diethyl ether ( εr = 4.3 ) shows that many ions strongly coordinate each other, which expresses itself in the fact that most of the salts are insoluble in such solvents: they form strong bonds in the form of a crystal lattice.

Ions in solids

In the solid state the measure of the strength of the interactions between the ions, the lattice energy. Depending case greater the distance of oppositely charged ions, the smaller the lattice energy. This can be demonstrated by the following table:

In the transition from monoatomic to polyatomic ions of the ionic radius is losing significance because only a few molecular ions are similarly constructed as highly symmetric single atomic ions. Especially with rather small, but very unbalanced ions calculated using the ionic radii soft lattice energies far from the experimental values ​​. To take this into account, the concept of thermochemical volume was introduced by Donald Jenkins.

Here, the volume of an ion is calculated from the volume of the elementary cell of an ion crystal and to calculate the lattice energy. As a semi-empirical method, the calculated lattice energies in many cases agreement with the experimental values.


The approach to the development of weakly coordinating ions is to distribute a low charge on the largest possible volume. This is the lattice energy in the solid state (and thus the interaction between oppositely charged ions) minimized. In addition, the ion must have a low polarizability, so a toy in the nearby counterion or solvent particles can not generate charge centers. These would otherwise turn cause dipole -dipole forces and thus lead to coordination.

The easy polarization is the reason why only limited large monatomic ions such as iodide or cesium act as weakly coordinating ions. Nowadays, the research therefore focuses on the production of very large monovalent ( single positively or negatively charged ) molecules.

Weakly coordinating anions

Weakly coordinating anions (English weak coordinating anion, WCA dec ) shall be implemented mainly by two different methods.

Covalently bound Gerüstanionen

One possibility is the construction of a polyatomic, negatively charged backbone having a spherical surface as possible, to which the charge is distributed. The atoms of the backbone are held together by strong covalent bonds.

Main representatives of this class are negatively charged carboranes such as [ CB11H12 ] -. By substitution of all hydrogen atoms the more stable carborane [1- R- CB11F11 ] could - (R = Me, Et ) are obtained, which is traded as far best weakly coordinating anion.

Stable Lewis acid - base complexes

The second approach is the development of highly stable complex anions of strong Lewis acids and Lewis bases. Of a cation of charge X and X 1 negatively charged ligands results in a complex having an overall charge of -1. Important for the stability of the complex is a strong coordinate bond the ligands to the central atom. So far, this highly charged cations such as B3 , Al3 , As5 , Sb 5 , Nb 5 , Y3 and La3 are used.

For the formation of a strong bond can be used as ligands in particular atoms with high electronegativity such as fluorine or oxygen. Anions such as tetrafluoroborate ( [BF4 ] - ), perchlorate ( [ ClO4 ] -) or hexafluoroantimonate ( [ SbF 6 ] - ) are already widely used in industry. It has been demonstrated that even such anions in nonpolar solvents coordinate comparatively strongly cations.

Therefore, ligands are increasingly used in research, which have bulky substituents, and a chemically inert surface. Significant representatives of alkyl and aryl ligands known as the [ BPh4 ] - ( Kalignost ). Derived from the corresponding alcohols to provide the alkoxy and Aryloxiliganden attached via the oxygen to the central atom.

To the surface of the ion to make invulnerable to chemicals (inert ), perfluorocarbons variants of ligands are used (see fluorocarbons and hydrofluorocarbons ). For example, the anion formed with perfluorinated tert- butanol ligand [ Al [ OC ( CF3) 3 ] 4] - in its properties similar to the previously commonly used industrially [ SbF 6 ] -.

  • Examples

[B ( C6F5 ) 4] -

[ Al {OC (CF3) 3} 4 ] -

Weakly coordinating cations

For the production of weakly coordinating cations are hardly any higher-level concepts. Thus, the development focuses mostly on the intended application, such as the stabilization of "naked" fluoride ions and other highly reactive anions.

Similarities of weakly coordinating cations is often the structure of bulky molecules containing a positively charged nitrogen, phosphorus or sulfur. The rest of the molecule is so constructed that they represent only a very weak Brønsted acids, so they can not eliminate protons, which would otherwise lead to decomposition of the cation. Thus, these molecules can be considered as salts of strong bases, a property which opens more applications.

The following table lists some of Cations in which weakly coordinated fluoride compounds are detected:

Thermodynamic properties

Measurements and calculations with the aid of the Born- Haber cycle process, it is possible to determine thermodynamic properties of compounds with large and weakly coordinating ions. The comparison of the properties for different ions is a measure of the quality of a particular ion.

If we calculate the lattice energy of solid compounds with very large weakly coordinating anions, one obtains - depending on the volume of the particles - very small values.

These values ​​compare quite well (eg, the fullerenes C60 and C70 ) with the sublimation of very heavy molecules:

The weakly coordinated compounds have therefore energies that are similar to molecules in the gas phase in the solid. In fact, the above-mentioned cation [Ag (S8 ) 2] were prior to rendering with a weakly coordinating anion is not known. Due to the fundamentally low lattice energy are also destabilizing effects as (as in this case sulfur) caused by very weakly bound ligands, reduced in their effect. Conversely, one can therefore say that the use of weakly coordinating ions stabilize such complexes.

Similar conditions can otherwise be achieved partly by isolation of the ions in a matrix or by using slightly modified zeolite.

Due to this low lattice energies, a number of other properties are obtained. Such salts are much more soluble in non-polar solvents having a low dielectric constant because the energy barrier to the solution of the ionic bond and by solvation of the solvent is lower than that of conventional salts. In addition, the lattice energy is already exceeded in some compounds, at room temperature, so that some salts are liquid at these low temperatures; However, common salt melts at temperatures above 800 ° C.


The applications are very diverse, so only a few examples are given in this section.


The cationic or anionic coordination is one of the most important process for the polymerization of alkenes. Frequently this positively charged metallocenes with Group IV elements such as titanium or zirconium are used. The activity of the catalyst strongly depends on the properties of the counterion. Only with the use of weakly coordinating ions such as [B ( C6F5 ) 4] - have the salts formed the required high activity. This increase in reactivity is attributed to the reduced demonstrably influence of the anion.

Lithium -ion batteries

One of the most well-known application areas within the electrochemistry are the lithium -ion batteries. To achieve a high battery voltage, the selection of the counter ion and the solvent is critical. The weaker the counter ion and the non-polar solvent, the selected, the higher the ionic mobility of the lithium ions, and thus the conductivity of the lithium electrolyte. Currently, in this area the most common Li [ PF6 ] - used, but there are already a number of patent applications for the use of other anions.

Ionic Liquids

Due to their unique properties, ionic liquids many uses as a solvent in organic chemistry, as electrolytes or for special technical applications. Based on the very low melting points and very low vapor pressures, different solubilities in polar and nonpolar solvents and, finally, due to their environmental impact is expected that ionic liquids will displace many organic solvents.

Organic Catalysis

In the catalytic organic chemistry sometimes be highly reactive cations such as Ag , Li or F- employed. The use of weakly coordinating ion allows it to run at lower temperatures the reactions. Thus, fluorination of aromatics, for example where chlorobenzene is converted directly into fluorobenzene, only with very reactive fluoride ion possible.