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Coordination Number - Overview, Definition, Factor, Examples, FAQs

Coordination Number - Overview, Definition, Factor, Examples, FAQs

Team Careers360Updated on 02 Jul 2025, 04:28 PM IST

The total number of atoms, ions, and molecules bonded to the central metal atom in a coordination complex is called the coordination number. Another term used for the coordination number is called ligancy. which means the number of atoms bonded to the central metal atom and it is generally called ligands.

This Story also Contains

  1. What is Coordination Number
  2. Factors That Affect Coordination Number
  3. How to Find the Coordination Number of Central Atom:
  4. Coordination Number Examples
  5. The geometry of Molecules Based on Coordination Number
  6. Some Solved Examples
Coordination Number - Overview, Definition, Factor, Examples, FAQs
coordination number(Image: shutterstock)

In this article, we cover the concept of coordination numbers which is fall under the Coordination Compounds. It is an important topic of class 12 for board exams and also for the Joint Entrance Examination (JEE Main) exam and the National Eligibility Entrance Test (NEET).

What is Coordination Number

Do you know how to define the Coordination number? it is defined as the total number of atoms, ions, or molecules attached to an atom in a specific molecule or crystal and is referred to as the coordination number of that atom. The coordination number of an atom is often referred to as its ligancy.

The ligands are the atoms, ions, or molecules that are attached to the center atom (or molecule/ion). When computing the coordination number of a central atom in a crystal, the legacy of molecules is computed differently.

According to the radius ratio, “The larger the charge, the smaller the ion becomes, limiting the number of groups that can coordinate.”

Commonly Asked Questions

Q: What is the coordination number in chemistry?
A:
The coordination number is the total number of points of attachment between a central metal atom or ion and the surrounding ligands in a coordination compound. It represents the number of donor atoms directly bonded to the central metal atom.
Q: How does coordination number differ from oxidation state?
A:
Coordination number refers to the number of ligand atoms directly bonded to the central metal atom, while oxidation state represents the degree of oxidation of an atom in a compound. They are distinct concepts: coordination number describes structural features, while oxidation state relates to electron distribution.
Q: How does the concept of coordination number relate to Lewis acid-base theory?
A:
In the context of Lewis acid-base theory, the coordination number represents the number of Lewis base donors (ligands) bonded to a Lewis acid (the metal center). The coordination process can be viewed as the formation of coordinate covalent bonds between the acid and base.
Q: What is meant by "effective coordination number"?
A:
The effective coordination number takes into account the actual spatial arrangement and bonding strengths of ligands. It can differ from the classical coordination number when some ligands are more strongly bound than others or when there are secondary interactions.
Q: What is meant by "coordination unsaturation"?
A:
Coordination unsaturation refers to a situation where a metal complex has fewer ligands than its preferred coordination number would suggest. This often results in highly reactive complexes that readily bind additional ligands or participate in catalytic processes.

Factors That Affect Coordination Number

Different types of forces hold the atoms together in these complexes and contribute to the observed coordination numbers. In fluoride complexes, the bonds to the extremely electronegative fluorine atoms are virtually ionic. Therefore increase in coordination number with fluoride ions of 4 to 6 to 7 for B3+, Fe3+, and Zr4+ is primarily due to the cation's increased size. This allows a growing number of fluoride ions to be packed around the center ion.

Also, check-

NCERT Exemplar Class 11th Chemistry SolutionsNCERT notes Class 12 Chemistry Chapter 9 Coordination compounds
NCERT Exemplar Class 12th Chemistry SolutionsNCERT Solutions for Class 12 Chemistry Chapter 9 Coordination compounds
NCERT Exemplar Solutions for All SubjectsNCERT Exemplar Class 12 Chemistry Solutions Chapter 9 Coordination compounds

Commonly Asked Questions

Q: What factors influence the coordination number of a metal complex?
A:
Several factors influence coordination number, including:
Q: Can coordination number change for the same metal in different compounds?
A:
Yes, the coordination number can vary for the same metal in different compounds. This depends on factors like the metal's oxidation state, the types of ligands present, and the overall geometry of the complex.
Q: How does the size of the central metal ion affect coordination number?
A:
Generally, larger metal ions can accommodate more ligands, leading to higher coordination numbers. Smaller metal ions tend to have lower coordination numbers due to limited space around them.
Q: What role does the electronic configuration of the metal play in determining coordination number?
A:
The electronic configuration influences the number of available orbitals for bonding with ligands. Metals with more available d orbitals can often form complexes with higher coordination numbers.
Q: How do monodentate and polydentate ligands affect coordination number?
A:
Monodentate ligands occupy one coordination site each, while polydentate ligands can occupy multiple sites. Polydentate ligands may lead to lower coordination numbers than expected because they take up more than one position around the metal center.

How to Find the Coordination Number of Central Atom:

The coordination number related to a given atom in polyatomic ions and molecules can be computed by counting the total number of atoms, it is bound. whether it is a single bond or a double/triple bond all are included to calculate coordination number. 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. This means the coordination number is 6.

Coordination Number

Because the center cobalt atom is connected to six different nitrogen atoms in the 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 cases, the coordination number of the center atom reflects the sum arrangement of neutrons around the atom in question. The total number of atoms that surround a specific atom in a crystal is determined by the atom's position in the crystal. In the case of crystals, there are two separate metrics of ligancy- the bulk coordination number and the surface coordination number. Also, students can find related topics below.

Also Check-

Commonly Asked Questions

Q: How is coordination number determined experimentally?
A:
Coordination number can be determined through various experimental techniques, including:
Q: How does the concept of coordination number extend to organometallic compounds?
A:
In organometallic compounds, coordination number includes both the number of ligand donor atoms and the hapticity of pi-bonded ligands. For example, in ferrocene (Fe(C5H5)2), each cyclopentadienyl ring contributes a coordination number of 5, resulting in a total coordination number of 10.
Q: What is the difference between coordination number in solution and in solid state?
A:
Coordination numbers in solution can be dynamic and may differ from those in the solid state. In solution, ligands can exchange more readily, and the coordination sphere can be influenced by solvent molecules. Solid-state structures are more rigid but can sometimes involve secondary interactions.
Q: What is the importance of coordination number in materials science?
A:
In materials science, coordination number is crucial for understanding and predicting crystal structures, material properties, and behavior. It affects properties such as melting point, hardness, and electrical conductivity in both metallic and ionic solids.
Q: What is the relationship between coordination number and the 18-electron rule?
A:
The 18-electron rule is a guideline for stable electron configurations in metal complexes. The coordination number contributes to achieving this electron count, with each ligand typically donating a pair of electrons. However, not all stable complexes follow this rule, especially for early transition metals.

Coordination Number Examples

A coordination number of a crystalline solid is the number of atoms, ions, or molecules that a central atom/ion has as its nearest neighbors. 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 are linked to six ligands, Cl and H2O, respectively.

[Cr(NH3)2Cl2Br2]– is another example. Because the total number of atoms/ions/molecules linked to Cr is discovered to be 6, the core atom Cr has coordination number 6. The bidentate ligand, the coordination number Co is 6 in the complex ion [Co(en)3 ]3+. Below students can find coordination numbers with some 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 and the compound's molecular shape is square planar.

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

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

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

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Commonly Asked Questions

Q: What is the most common coordination number for transition metal complexes?
A:
The most common coordination numbers for transition metal complexes are 4 and 6. However, coordination numbers can range from 2 to 12, with 8 and 9 also being relatively common for larger metal ions.
Q: Can coordination number be fractional?
A:
While coordination numbers are typically whole numbers, fractional coordination numbers can be used to describe average coordination environments in some materials, especially in solid-state chemistry or for compounds with mixed coordination environments.
Q: What is the highest known coordination number in a metal complex?
A:
The highest experimentally observed coordination number is 15, found in some thorium complexes. However, coordination numbers above 12 are extremely rare and usually involve very large metal ions and small ligands.
Q: How does the concept of coordination number apply to main group elements?
A:
While coordination number is most commonly discussed for transition metals, it also applies to main group elements. Main group elements often have lower and more consistent coordination numbers based on their valence electron configuration and size.
Q: What is the relationship between coordination number and denticity of ligands?
A:
Denticity refers to the number of donor atoms in a single ligand molecule. The coordination number must be equal to or greater than the sum of the denticities of all ligands in a complex. For example, two bidentate ligands could satisfy a coordination number of 4.

The geometry of Molecules Based on Coordination Number

We can calculate the geometry of a molecule using the coordination number. A list is given below with the corresponding geometry and coordination number.

Coordination number

Molecular Geometry

2

Linear

3

Trigonal Planar

3

T-shaped

3

Trigonal Pyramidal

4

Tetrahedral

4

Square Planar

5

Trigonal Bipyramidal

5

Square Pyramid

6

Octahedral

7

Pentagonal Bipyramidal

7

Capped Octahedron

8

Square Antiprism

8

Dodecahedron

8

Hexagonal Bipyramidal

9 and above

Other complex structures

There are different types of lattices like BCC, FCC, and many more. The abbreviation for BCC is body-centered cubic and The coordination number of the bcc atom is 8. The abbreviated of FCC is Face centered cell and the coordination number is 12. The abbreviation of CCP stands for cubic close-packed and the coordination number is 12. Similarly, hcp stands for hexagonal close-packed cell and the coordination number of hcp is 12. The coordination number of simple cubic seems to have a coordination number of 6 and each unit cell contains one atom.

NCERT Chemistry Notes :

Commonly Asked Questions

Q: What is the relationship between coordination number and complex geometry?
A:
The coordination number often determines the geometric arrangement of ligands around the central metal. For example, a coordination number of 4 typically results in tetrahedral or square planar geometry, while 6 usually leads to octahedral geometry.
Q: How does the concept of coordination number relate to crystal field theory?
A:
Crystal field theory uses coordination number to predict the splitting of d-orbitals in metal complexes. Different coordination numbers lead to different geometric arrangements of ligands, which in turn affect the energy levels of the metal's d-orbitals.
Q: How does coordination number affect the stability of metal complexes?
A:
Generally, complexes with coordination numbers that allow for stable geometric arrangements (like 4 or 6) tend to be more stable. However, stability also depends on other factors such as the nature of the metal and ligands, and the chelate effect.
Q: What is the significance of coordination number in bioinorganic chemistry?
A:
In bioinorganic chemistry, coordination number is crucial for understanding the function of metalloenzymes and other biological metal complexes. It helps explain how metals interact with proteins and other biomolecules to catalyze reactions or perform other biological functions.
Q: Can coordination number change during a chemical reaction?
A:
Yes, coordination number can change during reactions. This often occurs in substitution reactions where ligands are exchanged, or in redox reactions where the oxidation state of the metal changes, potentially altering its ability to coordinate ligands.

Some Solved Examples

Example. 1

The coordination number of a central metal atom in a complex is determined by

1) (correct)the number of ligands around a metal ion bonded by sigma bonds

2)the number of ligands around a metal ion bonded by pi-­bonds

3)the number of ligands around a metal ion bonded by sigma and pi-­bonds both

4)the number of only anionic ligands bonded to the metal ion.

Solution

The coordination number is the number of ligands that are bonded directly to metal by coordinate bonds. Thus, the coordination number is the number of ligands around a metal atom/ion bonded by sigma bonds.

Hence, the answer is the option (1).

Example. 2

The coordination number of HCPs is

1)8

2)10

3)6

4) (correct)12

Solution

As we learned in

Coordination number -

The total number of atoms touching a particular atom in the given unit cell is known as the coordination number and those atoms are known as the nearest neighbor.

- wherein

Unit cell Coordination number

  1. Primitive 6
  2. BCC 8
  3. FCC 12
  4. HCP 12

In HCP packing the coordination number of atoms is 12.6 in the Same plane, 3 in the upper plane, and 3 in the lower plane.

Hence, the answer is the option (4).

Example. 3

The coordination number of copper in Cuprammonium sulfate is

1)2

2)6

3) (correct)4

4)-4

Solution

In Cuprammonium sulfate [Cu(NH3)4]SO4 , there are four ammonia ligands bonded to the central metal ion through coordinate bonds.

Thus, the coordination number of Cu is 4.

Hence, the answer is the option (3).

Also, Refer To

NCERT SyllabusNCERT Book
NCERT SolutionsNCERT Exemplar Solutions
NCERT Revision NotesCBSE Board

Frequently Asked Questions (FAQs)

Q: What is the relationship between coordination number and the spectrochemical series?
A:
While the spectrochemical series primarily deals with ligand field strength, coordination number can affect how ligands interact with the metal orbitals. Different coordination numbers can lead to different crystal field splittings, even for the same ligands, thus influencing their position in the spectrochemical series.
Q: How does coordination number affect the redox properties of metal complexes?
A:
Coordination number can influence redox properties by affecting the electron density at the metal center and the stability of different oxidation states. Changes in coordination number during redox processes can also impact the overall reaction energetics.
Q: What is the importance of coordination number in understanding the behavior of metal ions in biological systems?
A:
In biological systems, coordination number helps explain metal ion selectivity, transport, and function in proteins and enzymes. It's crucial for understanding processes like oxygen binding in hemoglobin, zinc finger protein structure, and the activity of metalloenzymes.
Q: How does coordination number relate to the concept of hapticity in organometallic chemistry?
A:
Hapticity describes the number of contiguous atoms in a ligand bound to a metal center. While distinct from coordination number, hapticity contributes to the overall coordination number. For example, an η5-cyclopentadienyl ligand contributes 5 to the coordination number.
Q: What is the relationship between coordination number and the nephelauxetic effect?
A:
The nephelauxetic effect, which refers to the expansion of d-orbitals in metal complexes, is influenced by coordination number. Higher coordination numbers often lead to greater orbital expansion due to increased interaction between the metal and ligand orbitals.
Q: How does coordination number affect the kinetics of ligand substitution reactions?
A:
Coordination number influences substitution kinetics by affecting the accessibility of the metal center and the mechanism of ligand exchange. Higher coordination numbers may lead to more crowded metal centers, potentially slowing down substitution reactions or altering their mechanisms.
Q: What is the significance of coordination number in understanding the properties of lanthanide and actinide complexes?
A:
For lanthanides and actinides, which can exhibit high coordination numbers due to their large ionic radii, understanding coordination number is crucial for predicting and explaining their unique chemical and physical properties, including their behavior in separation processes and their luminescence.
Q: How does coordination number relate to the concept of steric number?
A:
Steric number, used in VSEPR theory, is the sum of the number of bonding pairs and lone pairs around a central atom. In metal complexes, the coordination number is often equal to the steric number, as most ligands contribute a bonding pair without additional lone pairs on the metal.
Q: How does coordination number affect the thermodynamic stability of metal complexes?
A:
Coordination number influences thermodynamic stability through factors like entropy and enthalpy. Higher coordination numbers often lead to more negative enthalpy of formation but can also result in decreased entropy. The balance of these effects determines overall stability.
Q: What is the importance of variable coordination numbers in bioinorganic catalysis?
A:
Variable coordination numbers in bioinorganic catalysts allow for flexible binding of substrates and cofactors. This variability is often key to the catalytic cycle, enabling processes like substrate binding, activation, and product release.
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