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# Coordination Number- Introduction, Geometry of Molecules Based on CN, Practice Problems, FAQs

Everyone loves playing sports and games. This is the question where no one says ‘NO’ Everyone likes to play or it is also very entertaining to watch games and sports either LIVE or on-screen. In our country, we all know the craze for cricket. If I am right, a small child even plays cricket on his street.

Here you all thought, why am I discussing this thing here? Do you know how many players play in a team in the game of cricket? Everyone says loudly that the number is 11. Have you seen a professional cricket match without playing 11? The answer is no. The selected team contained 15 members but only 11 played in the final match. If one may get injured while playing then the remaining 4 out of 15 may get a chance in place of that player. The same happens in chemistry also.

Complexes always combine with atoms to form stable compounds, whether maybe 2,3,4 or many more. They may gain or lose atoms to gain their stable state. These surrounding atoms arrange themselves in a maximum stable manner and are known as coordination numbers.

Let us study more about coordination numbers and study more complexes with different coordination numbers in detail!

TABLE OF CONTENT

## What is the coordination number?

The coordination number of the central atom in a specific molecule or crystal represents the overall number of atoms, ions, or molecules that are bound to it. A different term for an atom's coordination number is "ligancy."

An atom, an ion, or a molecule that is linked to another atom, molecule, or ion is said to be a Ligand. The determination of a molecule's ligancy varies depending on the coordination number of the central atom in the crystal.

## Coordination Number of a Central Atom

By measuring the total number of atoms an individual atom is bound to, whether through a single bond, a double bond, or a triple bond, one can determine the coordination number corresponding to that atom in polyatomic ions and molecules.

In $\left[Cr\left(N{H}_{3}{\right)}_{2}{Cl}_{2}{Br}_{2}{\right]}^{-}$, the coordination number of the core cation ${Cr}^{3+}$ can be calculated by counting the total number of atoms directly linked to the chromium ion, which is discovered to be six, using the example of the polyatomic ion provided by the formula $\left[Cr\left(N{H}_{3}{\right)}_{2}{Cl}_{2}{Br}_{2}{\right]}^{-}$.

Since the centre cobalt atom in the example below is connected to six separate nitrogen atoms, it can be seen that its coordination number is six.

In crystals' solid state formations, the linkages are less obvious. In these situations, the total number of atoms that are next to the central atom equals the value of the atom's coordination number.

Depending on the atom's position within the structure, a crystal will have a certain number of atoms all around it. In the case of crystals, the bulk coordination number and the surface coordination number are thus 2 different approaches to evaluate legacy.

Only the 𝝈 bonds between the ligands and the central atom are taken into consideration when calculating the coordination number of the central atom in coordination complexes. In this estimate,𝛑 bonds are not taken into consideration.

The core tungsten atom (represented by the symbol W) in the molecule tungsten hexacarbonyl, which has the chemical formula W(CO)6, has a coordination number of 6 despite the importance of 𝛑 bonding combined with 𝝈 bonding in such metal carbonyls.

## The geometry of Molecules Based on Coordination Number

For each value of the coordination number for the central atom, there are various conceivable geometric combinations. The following table lists these potential geometric forms.

 Coordination number Structure Representation 2 Linear 3 Trigonal planar, T-shaped trigonal pyramidal 4 Square planar or tetrahedral 5 Trigonal bipyramidal or square pyramid structures 6 Octahedral 7 Pentagonal bipyramidal,

## Practice Problems

Q1. In ${\mathbit{K}}_{2}\mathbit{Z}\mathbit{n}\left(\mathbit{O}\mathbit{H}{\right)}_{4}$Which of the following is the coordinating entity?

A. K+
B. Zn2+
C. OH-
D. [Zn(OH)4]2-

Solution:
A core metal atom bonded to a certain quantity of other atoms or molecules makes up a coordination entity. The opposing/counter ion, in this case, is K+, and the molecule linked to the central Zn atom is OH.

Q2. What is the coordination number of the complex $\left[\mathbit{C}\mathbit{o}\mathbit{C}\mathbit{l}\left(\mathbit{N}{\mathbit{H}}_{3}{\right)}_{5}{\right]}^{2+}$?

A. 6
B. 5
C. 4
D. 3

Solution: The complex $\left[\mathbit{C}\mathbit{o}\mathbit{C}\mathbit{l}\left(\mathbit{N}{\mathbit{H}}_{3}{\right)}_{5}{\right]}^{2+}$ has coordination number equal to 6. Here Co is bonded with 5 NH3 ligands and one Cl ligand directly. Hence, the coordination number is the sum of atoms surrounded by the core/central atom. So it comes out to be 6 for $CoCl\left(N{H}_{3}{\right)}_{5}$.

Q3. Which of the following has a characteristic property called a coordination number?

A.Oxidation number
B. Coordination entity
C. Central atom
D. Ligand

Solution:
The quantity of donor atoms to which a central metal ion is directly bonded is known as the coordination number, which is sometimes referred to as the secondary valence of the central metal ion in a complex. As a result, the coordination number is a property related to the central metal ion.

Q4. Predict the chromium's coordination number in ${\mathbit{K}}_{3}\left[\mathbit{C}\mathbit{r}\left({\mathbit{C}}_{2}{\mathbit{O}}_{4}{\right)}_{3}\right]$?

A. 3
B. 4
C. 5
D. 6

Solution: In the provided compound, the central ion has three groups same i.e oxalate groups connected to it, but because oxalate is a bidentate ligand, there are a total of six donor atoms through which the metal atom is directly linked. The coordination number of Cr in ${\mathbit{K}}_{3}\left[\mathbit{C}\mathbit{r}\left({\mathbit{C}}_{2}{\mathbit{O}}_{4}{\right)}_{3}\right]$ is, therefore 6.

## Frequently Asked Questions – FAQs

Q1. What do you understand by the term coordination compounds?
When neutral molecules or anions (referred to as ligands) connect to a central metal atom via coordinating covalent bonds in a Lewis acid-base reaction, the end product is a coordination complex (or ion). They at least have one set of electrons available to provide to a metal atom or ion. Lewis bases are considered as ligands.

Q2. Are secondary valency and coordination numbers the same?
Yes, The secondary valency is not ionizable and has a value equal to the coordination number. Negative ions or neutral substances can fill these needs. For instance, the coordination number of the metal Ni and its secondary valency are both four in [Ni(CO)4].

Q3. What do you understand when it is termed as a high coordination number?