What is Complex?

A metal ion is at the centre of a complex ion, which is surrounded by a number of other molecules or ions. These can be thought of as co-ordinate (dative covalent) bonds linked to the central ion. (In some circumstances, though, the bonding is more intricate.)

As we already discussed what is complex, now we will discuss what is the meaning of complex in detail.

What is the Meaning of Complex?

A covalent bond is established when two atoms share a pair of electrons. Because both nuclei are attracted to the electron pair, the atoms are stuck together. Each atom contributes one electron to the formation of a simple covalent connection, although this isn't always the case.

A covalent link (a shared pair of electrons) in which both electrons come from the same place is known as a co-ordinate bond (also known as a dative covalent bond).

Example of a Complex Substance:

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Nomenclature in Complex Chemistry

Complex substances have their own naming convention: The central metal ion is the name given to the metal. Ligands are the anions or molecules that are bonded to the metal. The coordination number refers to the number of ligand-binding sites on the metal ion. A coordinate covalent bond is a link between a metal ion and a ligand in which the ligand gives both electrons. Water, ammonia, and chloride ions are examples of simple ligands.

Because coordination complexes are so common, their structures and reactions are explained in a variety of ways, which can be misleading. The donor atom is the atom in a ligand that is bound to the central metal atom or ion. A metal ion is bound to numerous donor atoms in a typical complex, which can be the same or different. A polydentate (many bonded) ligands is a molecule or ion having many bonds to the core atom; ligands with 2, 3, 4, or even 6 connections to the central atom are frequent. These complexes are known as chelate complexes, and the process of forming them is known as chelation, complexation, and coordination.

Ion in Complex Chemistry

All of these have active lone pairs of electrons in the outer energy level in common. The metal ion is employed to establish coordinate bonds with them.

Lone pair donors are present in all ligands. To put it another way, all ligands act as Lewis bases.

A complex substance is one in which one or more ligands are datively linked to a core metal cation. A ligand is a substance that can create a dative covalent connection with a transition metal by using its lone pair of electrons. H2O, NH3, Cl-, OH-, and CN- are examples of ligands.

Complex Ion-Forming Cations Must Have Two Characteristics:

  1. The presence of a high charge density attracts electrons from ligands.

  2. Low-energy empty orbitals that can accept the lone pair of electrons from ligands.

D-block metal (transition metal) cations are small, have a high charge, and have low-energy vacant 3d and 4s orbitals. When their partially full d subshell absorbs donated electron pairs from other ions or molecules, they readily form complex ions. The coordination number of a cation is the maximum number of lone pairs of electrons it can receive. This number is determined by the cation's size and electronic configuration, as well as the ligand's size and charge. The most common coordination number is six, but 4 and 2 are also popular. [Fe(H2O)6]2+, [CoCl4]2-, [Cu(NH3)4(H2O),2]2+, and [V(H2O)6]3+ are examples of complex ions.

Complex Ion Equilibria

When two reactants are combined, the reaction usually does not finish. Rather, until a condition is reached in which the concentrations of the reactants and products remain constant, the reaction will produce products. At this stage, the rate of product production is the same as the rate of reaction formation. Chemical equilibrium exists between the reactants and products, and it will continue to exist until it is disrupted by an external force. The reaction's equilibrium constant (Kc) links the concentrations of the reactants and products. The reaction between the iron (III) ion and the thiocyanate ion, for example, is as follows:


The deep red thiocyanatoiron (III) ion ([ FeSCN]2+) is generated when Fe3+ and thiocyanate ion solutions are combined. The initial concentrations of Fe3+ and SCN will drop as a result of the reaction. One mole of Fe3+ and one mole of SCN will react for every mole of [ FeSCN]2+ produced. 

Complex Substance Equilibria

At a fixed temperature, the value of Kc remains constant. This indicates that Fe3+ and SCN mixes will react until the equation above is met. Regardless of the initial amounts of Fe3+ and SCN utilised, the Kc value will be the same. The appearance of the red colour — indicating the developing [FeSCN]2+ ion — can be measured using spectrophotometry to find Kc for this reaction experimentally. At 447 nm, the wavelength at which the red complex absorbs the maximum light, the amount of light absorbed by the complex is measured. The complex's absorbance (A) is proportional to its concentration (M) and can be directly measured using a spectrophotometer:

A = kM

The Beer-Lambert Law is a relationship between the amount of light absorbed and the concentration of the substance that absorbs the light, as well as the length of the path along which the light passes:

The observed absorbance (A) is proportional to the molar absorptivity constant (), route length (b), and molar concentration (c) of the absorbing species in this equation. The equation depicts the relationship between concentration and absorbance.

A = ϵbc

The coordination number refers to the total number of points of attachment to the centre element, which can range from 2 to 16 but is usually 6. In simple terms, the relative sizes of the metal ion and the ligands, as well as electronic parameters such as charge, which is reliant on the metal ion's electron configuration, determine the coordination number of a complex. The phrase ionic potential, which is defined as the charge to radius ratio (q/r), is used to explain these opposing effects.

Did You Know?

Electronic transitions caused by light absorption produce spectacular colours 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 excited by a photon to another d orbital of higher energy. Charge transfer is still possible in complexes with d0 or d10 configurations, even though 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). Electronic spectroscopy, also known as UV-Vis, can be used to observe these phenomena. Tanabe–Sugano diagrams can be used to assign d–d transitions to simple compounds with high symmetry. With the help of computation, these assignments are becoming more popular.

FAQs (Frequently Asked Questions)

1. What is The Electrical Neutrality of Atomic Nuclei?

Ans: When an atom contains the same amount of electrons as protons, it has the same number of negative (electrons) and positive (protons) electric charges (the protons). As a result, the atom's total electric charge is zero, and it is referred to as neutral.

2. How Does The Formation of Complex Ions Affect Solubility?

Ans: The creation of a complex ion with a high Kf can drastically improve the solubility of sparingly soluble salts. We could expect a salt-like AgCl to be significantly less soluble in a concentrated solution of KCl than in water due to the common ion effect.

3. Why are Complex Ions Coloured?

Ans: The energies of the d orbitals of the central ion are affected differently by different ligands. The greater the splitting, the more energy is required to move an electron from a lower to a higher group of orbitals. Greater energy relates to shorter wavelengths in terms of the colour of the light absorbed.