Owing to the multidimensional applications in the dictum of foremost branches of science, coordination chemistry is one of the liveliest fields of research in chemistry. The development of coordination chemistry occurred slowly throughout the 19th century; it was in the last two decades, with the work of Sophus Mads Jorgensen in Denmark and Alfred Werner in Switzerland, that the heart and soul of this discipline, especially the coordination chemistry of transition metal complexes took shape. Nevertheless, the contributions made by both chemists are massive but it was Werner, an impulsive and intuitive genius, who supplied the conceptual firepower that made an eternal impact in coordination chemistry. 

A coordination polyhedron is defined by the spatial arrangement of the ligand atoms/molecules directly attached to the central atom/ion. In the particular case of transition metal complexes, the most common structures accomplished are: octahedral, square planar, tetrahedral and square pyramidal geometries. 

It is established that the structural and functional properties of coordination compounds depend on the nature of the metal ion, donor atom, structure of the ligand and the metal-ligand interactions. The differences in extent and mode of interactions of different polyhedra of donor atoms are found to be the foremost cause for the versatile behaviors of complexes. Nitrogen, oxygen and sulphur are the most documented donor atoms. Nitrogen and oxygen are found to be the key donor atoms in most of the metallobiomolecules, whereas sulphur plays important part in construction of metalloproteins. The extent and mode of coordination of these donor atoms is unique, different from each other and are the function of atomic properties. It should also be noted that the structure of the organic core which bears the donor atoms also plays very important role in deciding the coordination behavior. 

The preparation of the coordination compound is two-step process. In the first step a suitable coordination cavity is constructed with the utility of various heterocyclic compounds having flexible chelating options. In the second step, an appropriate metal ion with previously documented applications is made to fit the cavity. The structural and functional characters of complex are established to be dependent on both the ligand skeleton and metal ion. 

Coordination compounds are found to act as the spawning ground for the evolution of vast areas of chemistry. In bioinorganic chemistry, which is a leading discipline at the interface of chemistry and biology, there has been a great exploration due to the coordination compounds. The construction of enormous number of motifs which can structurally and functionally mimic ‘metallobiomolecules’ is possibly achieved in domain of coordination chemistry, and these ‘models’ offer a great insight into better understanding of the nature and reactivity of active sites and possible reaction mechanisms. 

The use of coordination compounds in homogeneous catalysis provides a new pathway. The metal complexes act in different ways within the catalytic reaction; they bring the substrates together, activate the substrates by coordinating to the metal and lower the activation energy of the reaction between substrates. Therefore, enantioselective (asymmetric) synthesis, which is very recent advance in the synthetic organic chemistry, can be easily achieved. Along with the homogeneous catalysis, the coordination compounds are employed in small molecule activation, electrocatalysis, fuel cell application and chemical conversion of solar energy. 

Most of the major classes of pharmaceutical agents which are in current clinical use contain a number of coordination compounds. The utility of metal complexes as pharmaceutics has a special emphasis because of their ability to undergo biotransformation. Due to the target specific strategies and low toxicity profile, the metal complexes are the frontline drugs for various dangerous diseases, viz., AIDS, cancer, diabetes, etc., of this new age. Aspects of coordination compounds do obey the principles of green chemistry by which they are helpful in many features that could directly lead to the revolutionary targets in environmental chemistry. The synthesis and design of electrocatalysts to reduce abundant and inexpensive CO2 is the important task in lessening the greenhouse effect and in developing new sources of fuels. Along with these applications, coordination compounds also can make a great contribution in the development of solid-state chemistry and polymer chemistry..

Further Reading:

  1. Wikipedia and other online sources.
  2. Introduction to Coordination Chemistry, book by Geoffrey A. Lawrance, Willey publishers, 2010.
  3. Classic books: Comprehensive Coordination Chemistry I (1987) and Comprehensive Coordination Chemistry II (2003), Elsevier Science.
  4. Medicinal Applications of Coordination Chemistry, book by Chris J Jones and John R Thornback, RSC Publishing, 2007.

"This article is authored by Dr. Naveen V. Kulkarni, Department of Chemistry, School of Arts and Science, Amrita Vishwa Vidyapeetham, Amritapuri"

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