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Coordination Chemistry

The evolution metals form a huge number of complex compounds in that the metal atoms are link to a number of anions or neutral molecules. In modern terminology these compounds are said coordination compounds. The chemistry of coordination compounds is a significant and challenging area of modern inorganic chemistry.

New ideas of chemical linking and molecular structure have provided insights into the functioning of essential components of biological systems. Chlorophyll, haemoglobin and vitamin B12 are coordination compounds of magnesium, iron and cobalt correspondingly. Variety of metallurgical procedures, industrial catalysts and analytical reagents engage the employ of coordination compounds. Coordination compounds as well find many applications in electroplating, textile dyeing and medicinal chemistry.

Compounds as Na[Ag(CN)2] and Na2[Zn(CN)4]. These compounds are termed to as coordination compounds or complex compounds. Coordination compounds play a significant role in the chemical industry and in life itself. For instance, the ZieglerNatta catalyst that is utilized for polymerization of ethylene is a complex containing the metals aluminum and titanium. Metal complexes play significant role in biological systems.

For instance, chlorophyll, that is essential for photosynthesis in plants, is a magnesium compound and hemoglobin, which carries oxygen to animal cells, is an iron complex. Such are the compounds that contain a central atom or ion, usually a metal, surrounded via a number of ions or molecules. The complexes tend to retain their identity even in solution, even though partial dissociation may happen. Complex ion might be cationic, anionic or nonionic, depending on the sum of the charges of the central atom and the surrounding ions and molecules.

Coordination compounds were identified in eighteenth century. It was a mystery for the chemist, of those days to understand as to why a stable salt like CoCl3 reacts through fluctuating number of stable molecules or compounds these as ammonia to provide numerous new compounds: CoCl3 .6NH3, CoCl3 .5NH3 and CoCl3 .4NH3 ; and what are their structures? Such compounds differed from each other in their chloride ion reactivity. Conductivity measurements on solutions of such compounds showed that the number of ions present in solution for each compound is different. Several theories were proposed, but none could satisfactorily instance all the observable properties of such compounds and similar other series of compounds that had been prepared via then. It was only in the year 1893 that Werner put forward a set of ideas which are recognized as Werner's coordination theory, to explicate the nature of bonding in complexes. His theory has been a guiding principle in inorganic chemistry and in the concept of valence. The significant postulates of Werner's theory are

1. Metals exhibit 2 kinds of valence:

(a) Primary valence (ionizable)

(b) Secondary valence (non-ionizable).

Primary or ionizable valence is satisfied through negative ions and corresponds to oxidation state of the metal. The secondary or non-ionizable valence, which is satisfied by negative, positive or neutral groups, is equal to the coordination number of metal ion. Every metal tends to satisfy both its primary and secondary valence.

2. The secondary valence is expressed toward fixed positions in space for example this has spatial arrangement analogous to dissimilar coordination number.

Bonds in introductory chemistry are classically classified according to whether they are ionic or covalent in character. Coordinate covalent bonds are a 3rd classification. In this kind of bond, a lone pair of electrons from one chemical species is donated to an empty orbital on another chemical species to form the new bond. This is dissimilar from a covalent bond since both electrons come from one atom or molecule but are shared as in a typical covalent bond. Unlike an ionic bond, a coordinate covalent bond doesn't rely on formal electrostatic attraction between a cation and an anion to form. Ammonia-borane complexes are an instance of this kind of linking figure

1675_Ammonia-borane complexes.jpg

Fig: Ammonia-borane complexes

Above figure Ammonia-borane complexes contain a coordinate covalent bond where the lone pair of electrons on ammonia (NH3) is given to an unfilled p orbital on borane (BH3) to provide NH3BH3.

These complexes are generally termed to as donor acceptor complexes where ammonia would symbolize the donor and borane would signify the acceptor. Transition metal coordination complexes can be thought of in much the equivalent way except for that the evolution metal is the acceptor and the molecules through lone pairs of electrons are termed to as ligands. Ligands can be neutral molecules (sometimes termed to as L type ligands), anionic (sometimes termed to as X type ligands), or in several cases even cationic. The ligand electrons are provided into d orbitals on the metal to form the new coordinate covalent bonds (Figure).

932_coordinate covalent bonds.jpg

Fig: Coordinate covalent bonds

Figure: Formation of coordinate covalent bonds in transition metal complexes. M represents a transition metal.

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