Introduction

Coordination chemistry is the foundation of modern inorganic and bioinorganic chemistry, both of which have contributed immensely to the expansion of the chemical industry and medicine. The knowledge of coordination chemistry has provided insight into the mode of actions (kinetics and mechanisms) of biological molecules in living systems. Significant biological molecules such as vitamin B12, chlorophyll, haemoglobin and myoglobin are coordination compounds of cobalt, magnesium and iron. The comprehensive understanding of the mode of actions of such compound molecules has been made possible through the knowledge of coordination chemistry. Coordination chemistry has also donated to the expansion of textile industry where dyeing involves utilize of coordination compounds. The tremendous expansion in the petrochemical industry would not have been made possible with no catalyst design that needs the knowledge of coordination chemistry. From this background, the knowledge of coordination chemistry is inevitable to chemists if not all scientists.

Definition and recognition of coordination compounds

Coordination compounds are shaped via the reaction between Lewis acids and Lewis bases. By Definition, Lewis acids are electron pair acceptors while Lewis bases are electron pair donors. Thus a Lewis acid must have empty suitable orbitals to accommodate the donated electron pairs. The presence of empty appropriate orbitals in transition metals (Cu, Co, Fe and so on and several compounds (BF₃, BeCl₂  through empty p-orbital) and ions (H⁺) of major block elements makes them to act as Lewis acids. Nevertheless, the chemistry of coordination compounds is controlled to compounds in that the Lewis acid is a transition metal or d-block elements.

A molecule can function as a Lewis base provided it has heteroatom(s) with lone pair(s) on them. Examples of such molecules are H₂O, NH₃, CO etc, anions such as halides (F-, Cl-, Br-), cyanide (CN^-) are also Lewis bases. The chemical interaction between a Lewis acid and a

Lewis base leads to coordinate bond formation hence the product of the interaction is said coordination compound. By definition, coordination compound is complex formed when a central metal atom or ion is surrounded (coordinated) to a number of anions or molecules in such a way that the number of the coordinated anions or molecules exceeds the normal covalency of the central atom or ion. The compound is as well termed to as complex since on ionization, it exists as an independent species without dissociation, unlike normal or double salt that dissociates on ionization. Complexes are enclosed in square brackets to differentiate them from other kinds of salts 

NaCl_(aq)   →   Na⁺_(aq)  + Cl-_(aq)   (Normal Salt)

KAl(SO₄)_2(aq)   →   K+_(aq)  +Al³⁺ _(aq)  + 2SO₄^2-_(aq)   (Double Salt)

[Co(NH₃)₃Cl₃]_(aq)   →   Co(NH₃)₃Cl_3(aq)    (complex Salt)

Coordination Compounds (Complexes) 

Werner's contributions to coordination chemistry

Alfred Werner (1866-1919) became the 1^st Swiss Chemist to receive the Nobel Prize in Chemistry due to his contribution to coordination chemistry. He prepared, characterized and studied mutually physical and chemical properties of several coordination compounds via easy experimental techniques these as precipitation. From his discovering, he made the subsequent conclusions:

i. Central metal atom or ion in a compound possesses 2 kinds of valencies named Primary and secondary valencies

The primary valency is ionizable and can be satisfied by anions only. It can be considered as the oxidation state of the central metal.

iii. The secondary valency isn't ionizable and can be satisfied via both molecules and anions. It provides rise to the coordination number.

iv. The spatial arrangement of the anions and molecules satisfying the secondary valency determines the shape of the complex. The compound species is enclosed in square bracket whilst the anions satisfying only the primary valency lie outside the coordination sphere (square bracket). As we know that anions in the coordination sphere satisfied both primary and secondary valencies but the molecules only satisfy secondary valency. From Werner's postulates geometries have been assigned to complexes depend on the number of the secondary valencies   Complex, Primary and secondary valencies, possible shape.

Electrolyte and non-electrolyte complexes 

Through Precipitation of Chloride ions (Cl^-) using silver nitrate (AgNO₃) solution on compounds of cobalt through similar chemical composition (CoNH₃Cl₃), Werner was able to distinguish to different kind of complexes that he classified as non-electrolytes and electrolytes From his experiment, a complex enclosing chloride(s) which gave precipitate on reacting through AgNO₃ solution was said to be an electrolyte while non-electrolyte gave no precipitate. The precipitated chloride satisfied only primary valency for instance it was outside the coordination sphere while un-precipitated chloride satisfied both primary and secondary valencies for instance it was in the coordination sphere. Through development in technology, complexes containing other shapes of anions can now be classified as electrolyte or non-electrolyte by measuring their electrical conductivity. It is worthy to note that for complexes by net negative charge ([Fe(CN)₆]^4-), cations can balance out the charge resulting to electrolyte complexes (K₄[Fe(CN)₆]). Consequently a complex is said to be an electrolyte if it has counter ion (cation or anion) outside the coordination sphere while a composite through zero net charge is said non-electrolytes since no counter ion will be present.

Table: Electrolyte and Non-Electrolyte Complexes 

Ligands

Ligands are Lewis bases that coordinate to central metal atom or ion in a complex. They might be molecules with heteroatom (for example H₂O, NH₃, PPH₃ etc) having lone pair(s), anions (for instance CN-, F-, Cl-, SCN-), unsaturated hydrocarbons (H₂C=CH₂, C₄H₄), aromatics (C₆H₆) or macromolecules like proteins, vitamins, porphyrin and so on. Ligands can be group into different classes based on the number of donor atoms (sites) they possess. A ligand with one donor site site is called monodentate (for instance H₂O, NH₃, CN^-, and Cl^-). Bidentate ligands are those with two binding sites for instance ethylenediamine (H₂NCH₂CH₂NH₂), oxalate (C₂O₄)^2- and so on. These ligands are capable of forming ring structure (Chelate) with central metal.

Tridentate ligands possess three donor atoms for instance diethylenetriamine (H₂NCH₂CH₂NHCH₂CH₂NH₂). They have capacity to form two ring structures around the central metal. Ligands through 4 donor sites are said to be quadridentate for instance triethylenetetraamine (H₂NCH₂CH₂NHCH₂CH₂NHCH₂CH₂NH₂), they are capable of forming complexes through 4 ring structures. Polydentate ligands have many donor sites for instance EDTA (ethylenediaminetetraacetate). Another category of ligands are said ambidentate ligands since they contain 2 heteroatoms and any can be utilized to coordinate to metal depending on the reaction situations for instance SCN^- (it can either coordinate through S or N), NO₂ (it can coordinate through O or N) structures of Ligands 

Application and importance of coordination compounds

i. Inorganic Analysis

Coordination compounds are utilized in quantitative and qualitative analyses of metals. The presence of metal can be noticed via using aqueous ammonia to form complex of the metal if present. An instance is the deep blue colour given via aqueous solution of Cu²⁺ on addition of ammonia solution. Estimation of the quantity of a metal fo9r instance CU²⁺, Zn²⁺, Ni²⁺ etc can be carried out using EDTA solution in the presence of an appropriate indicator. This can be utilized to quantify the hardness of a specified water example caused Ca²⁺ and Mg²⁺.

ii. Purification of Metals

Metals these as Gold and Nickel can be purified via complexation reaction. Gold in a given sample can be complexed with CN^- to give [Au(CN)₂]^-. By reacting the complex through Zn, the Gold can be recovered in a pure state. As well Nickel purification involves formation of [Ni(CO)₄] from a specified impure sample. The complex can be simply decomposed to attain pure Nickel.  

Pure titanium metal is made via chlorination of the oxide TiO₂ to form the tetrahedral complex TiCl₄. This is then diminished in a redox reaction through magnesium metal to yield free titanium metal as a powder. To give a continuous loop of reagents, the MgCl₂ as well shaped in this diminution step is electrolyzed to generate chlorine and magnesium metal. Through the chlorine and magnesium re-used fully, this is a good instance of an industrial process by little environmental pollution.

iii. Industry

In the petrochemical industry, Nickel, Platinum and Rhodium complexes are utilized as catalyst in hydrogenation reaction, Cobalt and rhodium complexes are employed in hydroformylation (conversion of alkene to aldehyde) reaction. EDTA is employed in production of cream, food and so on as preservative to trap any metal that might reason discoloration of the products. Brilliant and intense colours of many compounds make them of huge value as dyes and pigments. Instances are copper phthalocyanine and Prussian blue.

iv. Medicine

In medicine, metal poison, these as Lead poison, can be treated via the employ of ligands as EDTA as sequestering agent to form harmless complex by the toxic metal. Main breakthrough have been reported in the employ of Cis-Platin, [PtCl₂(NH₃)₂] and other relateddrugs in cancer treatments. Technetium complexes are utilized in imaging of internal organ in living organisms. Auranofin (gold complex) is employed in treatment of arthritis. Budotitane and titanocene dichloride (anticancer drugs) are titanium complexes.

v. Important Biological Complexes 

Photosynthesis is made possible in green plant due to the presence of Magnesium complex chlorophyll employed to trap the required light energy. Iron complex said Haemoglobin assists in transportation of Oxygen with the body while Myoglobin another Iron complex assists to store Oxygen.  Such are several of the areas where complexes play important role in our lives. 

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