Co-Ordination Compound

A coordination complex is made up of a core atom or ion, which is usually metallic and is referred to as the coordination centre, and an array of attached molecules or ions, which are referred to as ligands or complexing agents. Coordination complexes are found in many metal-containing compounds, particularly those incorporating transition metals. [Cu(NH3)4] are two examples. Ni(CO)4; SO4.

Coordination compounds include vitamin B12, haemoglobin and chlorophyll, as well as dyes, pigments, and catalysts used in the synthesis of organic molecules.

Living organisms require naturally occurring coordination molecules. In biological systems, metal complexes play a variety of vital roles. Metal complexes (metalloenzymes) are found in many enzymes, which are naturally occurring catalysts that regulate biological processes. For example, carboxypeptidase, a hydrolytic enzyme crucial to indigestion, has a zinc ion coordinated to multiple amino acid residues of the protein. Another enzyme that incorporates iron-porphyrin complexes is catalase, which is an excellent catalyst for the breakdown of hydrogen peroxide. Haemoglobin also contains iron-porphyrin complexes, and its function as an oxygen transporter is dependent on the iron atoms’ capacity to reversibly coordinate oxygen molecules.

Electronic transitions triggered by light absorption provide beautiful hues in transition metal complexes. As a result, they’re frequently used as pigments. d  –d transitions or charge transfer bands are the most common transitions associated with coloured metal complexes. d–d transitions occur only for partially-filled d-orbital complexes (d1–9) because an electron in a d orbital on the metal is stimulated by a photon to another d orbital of higher energy. Charge transfer is nevertheless conceivable in complexes with d0 or d10 configurations, even when d–d transitions are not. An electron is promoted from a metal-based orbital to an empty ligand-based orbital in a charge transfer band (metal-to-ligand charge transfer or MLCT). Excitation of an electron in a ligand-based orbital into an empty metal-based orbital also happens (ligand-to-metal charge transfer or LMCT).

Properties of Coordination Complexes

The general properties of coordination compounds are discussed in this subsection.

• The existence of unpaired electrons in the coordination compounds created by the transition elements causes the coordination compounds to be coloured as a result of the absorption of light during their electronic transitions. Examples of coordination compounds including iron(III), such as iron(II) complexes, are green or pale green in colour, whereas coordination compounds containing iron(II) are brown or yellowish-brown.

• In cases when the coordination centre is a metal, the related coordination complexes exhibit magnetism as a result of the existence of unpaired electrons in the coordination centre.

• Coordination compounds are capable of exhibiting a wide range of chemical reactivity. In both inner-sphere electron transfer reactions and outer-sphere electron transfer reactions, they can be found as byproducts.
• It is possible for complex molecules to act as catalysts or as catalysts and stoichiometric catalysts in the transformation of molecules, depending on the ligands present bonds.

Complexes show a variety of possible reactivities:

Electron transfers

Inner and outer sphere electron transfers are two different processes for electron transmission between metal ions. A bridging ligand acts as a channel for ET in an inner sphere reaction.

(Degenerate) ligand exchange

The rate of degenerate ligand exchange is a key measure of reactivity. Labile complexes are those in which the ligands are released and recover quickly. Thermodynamically, such labile compounds can be quite stable. Labile metal complexes typically contain low charge (Na+), antibonding electrons in d-orbitals with respect to the ligands (Zn2+), or lack covalency (Ln3+, where Ln is any lanthanide). When high-spin vs. low-spin configurations are available, the lability of a metal complex is likewise affected. As a result, high-spin Fe(II) and Co(III) form labile complexes, while low-spin equivalents are inactive. Because of its high formal oxidation state, absence of electrons in M–L antibonding orbitals, and some “ligand field stabilisation” associated with the d3 configuration, Cr(III) can only exist in the low-spin state (quartet), which is inert.

Associative processes

Unfilled or half-filled orbitals in complexes often indicate the propensity to react with substrates. Because most substrates contain a singlet ground state, or lone electron pairs (e.g., water, amines, and ethers), they require an empty orbital to react with a metal centre. Some substrates (for example, molecular oxygen) have a triplet ground state, which causes metals with half-filled orbitals to react with them (it should be noted that the dioxygen molecule also has lone pairs, so it can react as a ‘regular’ Lewis base). The metal can aid in molecular transformations or be utilised as a sensor if the ligands around it are carefully chosen.

Application of coordination compounds

In metallurgy,

(a) The production of cyanide complexes, such as dicyanoargentate and dicyanoaurate, allows noble metals like silver and gold to be recovered from their ore.

(b) The formation and subsequent disintegration of metal coordination complexes can be used to purify metals.

Nickel can be refined by reacting with carbon monoxide to generate tetracarbonyl Nickel (0), a volatile compound that can be thermally destroyed to yield pure Nickel.

In biological systems

(a) Haemoglobin, the red pigment in blood that serves as an oxygen carrier, is an iron coordination complex.

(b) Metal complexes are found in a large number of enzymes that govern biological processes.

Carboxypeptidase is a protease enzyme that hydrolyzes proteins and contains a zinc ion that is covalently bound to many amino acid residues.

In industrial processes 

(a) The Ziegler-Natta catalyst, which is a mixture of titanium tetrachloride and triethyl aluminium, is utilised in the polymerisation of ethene.

(b) A complex metal catalyst is used in the hydrogenation of alkenes.

In the field of analytical chemistry

(a) Complex formation is critical in the identification and separation of most inorganic ions using qualitative methods of investigation.

(A deep blue complex soluble in water is generated when copper sulphate solution is combined with aqueous ammonia.) The cupric ions in the salt are detected using this process.

(b) Titration with the sodium salt of EDTA is used to determine the hardness of water; the calcium and magnesium ions in hard water form stable complexes called magnesium EDTA and calcium EDTA. Because the stability constants of magnesium and calcium EDTA complexes differ, these ions can be calculated selectively.

Conclusion

The FeCl4- ion and CrCl3  6NH3 are called coordination compounds because they contain ions or molecules that are linked, or coordinated, to a transition metal. Because they are Lewis acid-base complexes, they are also known as complex ions or coordination complexes. Ligands are the ions or molecules that bind to transition-metal ions to generate these complexes (from Latin, “to tie or bind”). The coordination number refers to the number of ligands attached to the transition metal ion. A coordination complex is a chemical structure in which a core metal atom is surrounded by nonmetal atoms or groups of atoms, known as ligands, that are chemically linked to it. Vitamin B12, haemoglobin, and chlorophyll, as well as dyes and pigments and catalysts employed in the synthesis of organic substances, are examples of coordination compounds. Coordination compounds are used in a variety of fields today, including metallurgy, medicinal chelating agents, chemical analysis, catalysis, and detergents. Complex ions with a metal ion core and ligands attached via coordinate covalent bonds are known as coordination compounds.