In 1913, Werner, a Nobel Prize winner and Swiss chemist, broke through the traditional concept of chemical bonds and first proposed the basic viewpoints of coordination bonds, coordination numbers and coordination complexes. From the perspective of stereochemistry, the isomeric phenomena and spatial configuration of complexes were revealed, and the modern coordination chemistry theory was established. Subsequently, Lewis published the famous electronic pair theory in 1916, which became an important milestone in understanding the structure and properties of coordination complexes. The traditional theory holds that the complexes are formed by the ligand and the central atom combined by coordination bond. Ligands are ions (or molecules) that can give lone pair electrons or a certain number of delocalized electrons. Central atoms are atoms (or ions) that accept lone pair electrons or delocalized electrons. Coordination bond is the binding force between ligand and central atoms.
Relationship between coordination number and configuration
There are various basic configurations of coordination complexes, the most common are octahedral and tetrahedral. The former is like [Fe (CN)6]4-, and the latter is like [Ni (CO)4]. There are also square planar configurations such as [Cu (NH3)4]2+ and [Cu (H2O)4]2+. The configuration of the coordination complex is determined by the coordination number, which is the number of coordination atoms around the central atom of the complex. The relationship between coordination number and configuration is shown in Table 1.
Table 1. Coordination numbers and configurations of different coordination complexes
|4||Square planar||Pt (NH3)2Cl2|
|5||Trigonal bipyramid||Fe (CO)5|
The above is only the ideal configuration of the coordination complex. In practice, the configuration of coordination complex is often distorted due to steric effect, electronic effect, or ligand type.
The main property of coordination complexes is stability. Generally, the stability mainly refers to the thermal stability and whether the coordination complex is easy to ionize its components (central atoms and ligands) in solution. The stability of the coordination complex in solution is related to the radius of the central atom, its charge, and its position in the periodic table. For example, transition metals have high nuclear charge, small radius, empty d orbitals and free d electrons, which are easy to accept the electronic pairs of ligands and feedback d electrons to ligands. Therefore, they can form stable complexes. Alkali and alkaline earth metals are exactly the opposite of transition metals, which have a low polarizability and a more unique inert gas structure. Therefore, the ability to form complexes is poor, which ultimately leads to the poor stability of their complexes.
Recently, the design and synthesis of ligand complexes have attracted great interest from researchers. They have a wide range of applications due to their fascinating structures and flexible redox properties. The main fields of application are as follows:
- In the field of catalysis
It is known that many coordination complexes could catalyze some special organic or polymerization reactions to some extent. This means that the coordination complexes can be used as efficient and high-quality catalysts.
- In the field of biomedical
Natural metal complexes are involved in a plenty of biological mechanisms among which photosynthesis, transport of oxygen in blood, coordination of some metabolic processes, pathological states, enzymatic reactions, etc. Therefore, coordination complexes have potential applications in the biomedical field.
- In the field of materials
Coordination complexes are very promising candidates for development of molecular optoelectronic materials and devices due to their diverse structural, electronic, and optical properties, such as a narrow-band luminescence with a high quantum yield and structurally variable ligands.
- In the field of sensing
Some coordination complexes have properties such as high luminescence quantum yields, long phosphorescence lifetimes, and large Stokes shifts. These properties play an important role in the sensing field and constantly innovate the sensing technology.
- In the field of analysis
The formation of coordination complexes amplifies the differences in properties between metal ions, e.g., colour, solubility and stability all vary considerably depending on how the complexes are formed. Therefore, coordination complexes are widely used in the field of analysis.
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