Coordination Number

In chemistry, the coordination number plays a vital role. We can say that the coordination number is the number of ligands attached to the central ion. When talking about coordination numbers, they are almost between two and nine. The number of bonds formed is determined by the metal ion’s size, charge, electron configuration, and ligands. The chemistry of complexes is mainly dominated by interactions between s and p molecular orbitals of the ligands and the d orbitals of the metal ions. The metal’s s, p, and d orbitals can always hold 18 electrons. The maximum coordination number for a specific metal is thus related to the electronic configuration of the metal ion (precisely, the number of empty orbitals) and the size ratio of the ligands to the metal ion. Large metals and tiny ligands cause high coordination numbers (e.g., [Mo(CN)8]4−).

What is the coordination number? 

Coordinate compounds have a central metal atom surrounded by ligands via a coordinate or dative bond. In this case, the central metal functions as a Lewis acid, accepting electrons from ligands that function as Lewis bases. Alfred Werner was the first to offer his perspective on coordination compounds. These compounds retain their identity in crystal lattice and solution or molten states. The coordination number is the number of spheres (atoms, molecules, or ions) immediately encircling a single sphere in a crystal. In a face-centred cubic tight packing configuration, the coordination number is 12. In a three-dimensional structure, each sphere has contact with six spheres in its layer and three spheres in each of the bottom and higher layers.

Examples of Coordination Numbers

• Carbon has a coordination number of 4 in a methane (CH4) molecule because it is bonded to four hydrogen atoms.
• Each carbon in ethylene (H2C=CH2) has a coordination number of 3, with each C bound to 2H + 1C for a total of 3 atoms.
• A diamond has a coordination number of 4 because each carbon atom resides in the centre of a regular tetrahedron made by four carbon atoms.

Obtaining the Coordination Number

Here are the steps for determining a coordination compound’s coordination number.

• Determine the chemical formula’s centre atom. Most often a transition metal.
• Find the atom, molecule, or ion closest to the centre metal atom. 
• To do so, look for the molecule or ion next to the metal symbol in the coordination compound’s chemical formula. 
• If the centre atom is in the centre of the formula, it will be surrounded by atoms/molecules/ions on both sides.
• Add the atoms of the closest atom/molecule/ions. Even if the centre atom is only connected to one other element, the formula must include the number of atoms of that element.

Coordination Number Geometry

When discussing the coordination number in geometry, there are multiple possible geometric configurations for most coordination numbers.

• When we talk about Coordination Number 2, it’s known as linear.
• When we study Coordination Number 3, it is known as trigonal planar (e.g., CO32-), the trigonal pyramid is T-shaped.
• Coordination Number 4 is known as tetrahedral, square planar.
• When we study the Coordination Number  5, it is known as the square pyramid (e.g., oxovanadium salts, vanadyl VO2+), and the trigonal bipyramid.

Ligands 

The ligands are mostly  neutral molecules or ions bound to the central atom or ion in the coordination entity and are known as ligands. Ligands are typically donors; however, pi bonding in the molecular orbital or vacant d orbital, also known as pi acid ligand and pi base metal, allows ligands to absorb electrons from metal.

Homoleptic and heteroleptic complexes are two types of complexes. Metal surrounded by the same type of ligands is considered homoleptic, while metal surrounded by two or more types of ligands is considered heteroleptic.

Monodentate ligand 

There is only 1 donor site in a monodentate ligand — Ammonia (NH3), Carbonyl (CO), Phosphine (PH3).

Bidentate ligand

Here, it is a 2 donor site in the ligand. Oxalate (C2O4)2- is a good example.

Polydentate ligand

There are more than two donor atoms in a polydentate ligand. EDTA (Ethylene diamine tetraacetate) is an example of a hexadentate ligand.

Conclusion

In this article, we learned about the coordination number in chemistry. Coordinate compounds have a central metal atom surrounded by ligands via a coordinate or dative bond. In this case, the central metal functions as a Lewis acid, accepting electrons from ligands that function as Lewis bases. The complex ion of the coordination compound contains the central metal atom/ion and the ligands attached to it. Or we can say they enclose coordination entities collectively known as coordination and are usually enclosed in square brackets. It also states the net charge an entity has, which is enclosed inside the brackets. The other ionisable ions, written outside the brackets, are called counter ions.