What is the coordination number of metal in \(\left[\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}\right] ?\) (a) 3 (b) 4 (c) 5 (d) 6

Complex compounds are chemical species formed by a central metal atom or ion bonded to one or more molecules or ions called ligands. These ligands donate pairs of electrons to the metal, forming coordination bonds, which are a type of covalent bond.

In the complex \[\mathrm{Co}(\mathrm{en})_{2}\mathrm{Cl}_{2}\], Cobalt (Co) acts as the central metal ion, surrounded by ligands – in this case, ethylenediamine (en) and chloride ions (Cl). The entire complex can be charged or neutral, and in this instance, it's a neutral compound.

Understanding the structure and bonding of complex compounds is crucial in various fields such as chemistry and biochemistry, where they play significant roles in catalysis, transport, and structural functions.

The term 'dentate' refers to the number of donor sites a ligand has that can bind to a central metal ion in a complex compound. Ligands can be classified based on their dentate:

• Unidentate: Ligands with one donor atom.
• Bidentate: Ligands with two donor atoms.
• Tridentate and higher: Ligands with three or more donor atoms.

The nature of the ligand affects the overall shape and properties of the complex. For example, ethylenediamine (en), due to its two nitrogen atoms which both donate a pair of electrons, is considered a bidentate ligand. This ability to bind through two sites allows ethylenediamine to form more stable chelate complexes compared to similar unidentate ligands.

A bidentate ligand is a flexible ligand that can form two bonds with a single metal ion at distinct locations. Ethylenediamine (en), which was mentioned in our exercise, is a classic example of a bidentate ligand.

Due to their ability to hold onto the central metal atom or ion at two points, bidentate ligands form chelates, which are cyclic structures that tend to be more stable than complexes formed with the same quantity of unidentate ligands. This increased stability is due to the chelate effect, which is a special case of the entropy effect and is crucial for the formation of such stable compounds in nature and industry.