Coordination number

Introduction

Transitional elements/metals (mainly the d-block elements) give birth to a vast number of complex compounds in which the metal atoms are coupled to numerous anions (negatively charged ions) or neutral molecules. The coordination number is one of the most basic notions in Coordination Compound.

Coordination Numbers 

The coordination number (CN) of a metal ion/atom in a given complex can be defined as the number of ligand donor atoms to which the metal is directly bonded, or as the number of central atoms or ions that a central atom or ion holds as its nearest neighbours in a complex, coordination compound, or crystal form. Ligands are those ions (charged) or molecules (neutral)that are  attached to the coordination compound’s core atom/ion.

Examples of Coordination Numbers

As previously stated, the coordination number of a crystalline solid is the number of atoms, ions, or molecules that a central atom/ion retains as its nearest neighbours in the crystalline solid or coordination compound. According to our observations, magnesium has a strong coordination number of 6 and a strong affinity for water or oxygen-containing ligands.

For example, the coordination numbers of Pt and Fe in the complex ions [PtCl6]2-and [Fe(H2O)6]2+ are 6 and 6, respectively. Pt and Fe have been linked to six monodentate ligands, Cl–and H2O, in this case.

[Cr(NH3)2Cl2Br2]– is another example.  The total number of atoms/ions/molecules linked to Cr is discovered to be 6. The core atom of Cr has coordination number 6 once more. As we know en (ethylenediamine) is a bidentate ligand, the coordination number Co is 6 in the complex ion [Co(en)3]3+.

The metal-ligand connections in some compounds may not all be at the same distance. In some circumstances, a variant definition of coordination number is employed, which includes atoms that are separated from their nearest neighbours by a greater distance. The structure of several metals is irregular. The shape of many chemical compounds are twisted. Otherwise sodium chloride, which has cubic close-packed chloride ions, arsenic anions have hexagonal close-packed chloride ions.

A Central Atom’s Coordination Number

Coordinate bonds connect the ligands to the central metal atom/ion in a coordination molecule. To determine the coordination of the core metal atom/ion, we must first determine the total number of coordinate bonds formed by all ligands with the metal atom/ion.

Let’s look at an example:

[Pt(NH3)2Cl2] is a chemical that contains platinum.

It’s apparent that Pt has two coordinate bonds with NH3 and two with Cl in this structure. The total number of coordinating bonds formed by all ligands is four. As a result, the coordination number (CN) of [Pt(NH3)2Cl2]is 4.

Using the polyatomic ion [Cr(NH3)2Cl2Br2]– as an example. The coordination number of the core cation (Cr3+) can be calculated by counting the total number of atoms linked to the chromium atom, which is 6.

Because the center cobalt atom is connected to six different nitrogen atoms in the given  example above.The coordination number of the central cobalt atom is six.The bonds between crystals are less obvious in their solid state forms. In such instances, the value of the central atom’s coordination number equals the total number of atoms surrounding the atom.The total number of atoms that surround a specific atom in a crystal is determined by the atom’s position in that crystal. In the cases of crystals, there are two different types of ligancy: the bulk coordination number and the surface coordination number.

Only the sigma bonds between the ligands and the central atom are counted in the computation of the coordination number of the central atom in coordination complexes. In this calculation, Pi bonds are ignored.

Despite the importance of pi bonding coupled with sigma bonding in such metal carbonyls, the coordination number of the central tungsten atom (denoted by the symbol W) in the compound tungsten hexacarbonyl, represented by the chemical formulaW(CO)6, is 6.

Molecule Geometry Based on Coordination Number

The molecular geometry of the coordination complex can be deduced using the coordination number of the central metal atom/ion. The geometrical pattern is a polyhedron, with the polyhedron’s vertices corresponding to the ligands’ coordinating atoms’ centres.

The molecular geometry of coordination compounds is provided as follows based on the coordination number (CN):

Here are a few examples:

[Ag(NH3)2]+, where Ag has a coordination number of 2 and the compound’s molecular shape is linear.

[NiCl4]2, where Ni has a coordination number of 4. The compound’s molecular shape is square planar.

[CoCl6]3is a chemical with the coordination number 6. It is  an octahedral molecular shape.

The molecular geometry of [ZrF7]3is pentagonal bipyramid, with Zr having coordination number 7.

[CoCl5]2is a chemical with the coordination number 5 and a trigonal bipyramidal molecular shape.

Molecule Geometry Based on Coordination Number

For each value of the coordination number for the centre atom, there are various geometric combinations. Below is a list of possible geometric shapes.

As a result, there are many geometric structures that can be created with varied coordination numbers of a central atom that are addressed above.

Conclusion 

Hence we can conclude that coordination number is important in every aspects of chemical science.The influence of the relative size of the coordinated molecule and the central ion is important in hydrated cations, in which the coordinating bond is mainly the electrostatic attraction of the central cation and the negative end (oxygen atom) of the water molecule.