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Oxidation number of Elements in Coordination Compounds- Coordination Sphere, Coordination Entity, Counter-ion, Homoleptic and Heteroleptic Complex, Oxidation Number, Coordination Number, Practice Problems, FAQs

Oxidation number of Elements in Coordination Compounds- Coordination Sphere, Coordination Entity, Counter-ion, Homoleptic and Heteroleptic Complex, Oxidation Number, Coordination Number, Practice Problems, FAQs

Any entity except atoms are made up of more than one unit. We all know about the different parts or constituents of our body. 

Similarly, a coordination complex/compound is made up of many subparts- neutral, cationic, anionic groups, some within brackets some outside etc.
You will understand in detail about them.

Table of Contents

  • Central Atom
  • Ligands
  • Coordination Sphere
  • Coordination Entity
  • Counter Ion
  • Homoleptic and Heteroleptic Complex
  • Coordination number
  • Oxidation number
  • Coordination number and Oxidation Number/state
  • Practice Problems
  • Frequently Asked Questions

Central Atom

The central atoms and ions, are the atoms and ions to which a specific number of atoms, molecules, or ions called as ligands are linked.

[CoCl3(NH3)3] - Central Atom - Co (Cobalt) 

[Fe(CN)6]4- - Central Atom - Fe (Iron)


Ligands are atoms, molecules, or ions that are attached to the coordination centre or the central atom/ion. These ligands can be simple ions or neutral molecules (like Cl-,NH3), or it can be rather big compounds like ethane-1,2-diamine (H2NCH2CH2NH2).

[CoCl3(NH3)3 - Ligands - Cl-,NH3

[Fe(CN)6]4- - Ligands - CN-

Coordination Sphere

The non-ionizable portion of a complex chemical that is made up of a core transition metal atom or ion and the surrounding ligand atoms or groups is referred as coordination sphere. The coordination sphere is placed within square brackets with the charge of the complex-ion as a superscript outside the bracket.

[Co(NH3)5Cl]2+ and [Fe(CN)6]4- are two examples of coordination spheres.

Coordination Entity 

A coordination entity contains the central ions or atoms and the ligands -the number of atoms, molecules, or ions attached to the present in the coordination sphere.

It is obtained by removing the square brackets and the charge of the coordination sphere

K4[Fe(CN)6] - is the coordination compound .

[Fe(CN)6]4- - is the coordination sphere of the complex

Fe(CN)6 - is the coordination entity of the complex

Counter Ion

The species or the ions outside the square bracket (coordination sphere) of a coordination compound are known as counter ions. 

[CoCl2(NH3)4]Br - Counter Ion - Br-

[Co(NH3)5Cl]SO4 - Counter Ion - SO42-

A counter ions are ionizable and break away from the coordination complex in aqueous solution.

Homoleptic and heteroleptic complexes 

The coordination complex is called a homoleptic complex when the coordination centre is bound to only one type of electron pair donating ligand group, such as [Cu(CN)4]3-.-in other words attached to the same ligands.

The coordination compound is a heteroleptic complex when the central atom is bound to many different types of ligands, such as [CoCl3(NH3)3].

Coordination Number

The total number of coordinate bonds through which the ligands are bound to the coordination centres is referred to as the coordination number of the central atom or the coordination complex.

Coordination complex

Coordination centre

coordination number 













Oxidation Number

It is the charge associated with the central atom after all the ligands and the electron pairs shared with the central atom by the ligands are removed.

How to calculate the oxidation number?

All assumptions used to calculate the oxidation state of an element in a compound are applicable here also.

The sum of charges of all constitutes elements and groups in the coordination complex sphere should be equal to the total charge of the coordination sphere.

Coordination complex


Ligand attached and its oxidation number

Oxidation Number of the central metal(x)



NH3 - 0


X = +2



NH3 - 0

Cl - -1





CO - 0





Cl - -1






CN - -1






NH3 - 0

Cl - -1

x+5(0)-1 = +2




OH -1

x+4(-1)= -2




C2O4 - -2

x+3(-2)= -3

X - 6= -3


Coordination number and Oxidation number


Coordination Number

Oxidation number/state



Equal to the total number of coordinate bonds only between the central atom/ion and the ligands.

Equal to the charge possessed by the ion after removing all the ligands and the electron pairs shared with the central atom by the ligands are removed.


Belongs to

Central atom or the whole complex

The central atom only

Integer nature

Positive whole number

Positive or zero

Whole number or fractional


Only attached ligands

Bonded both ligands and electron pairs

Practice Problems

Q1. What is the coordination number of Aluminum in a given complex K3[Al(C2O4)3

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

Solution: As the ligand is bidentate, it means one ligand will able to donate from two atoms. There are three oxalate ligands given. Hence, in total six donor atoms are present around the aluminum for donation. Hence the coordination number is 6.

The correct answer is (D).

Q2. Calculate the oxidation number of the central atom

(A) [Co(NH3)36]Cl3
(B) K3[Fe(C2O4)3]
(C) [Pt(NH3)2Cl(NH2CH3)]Cl
(D) [Co(NH3)5Cl]Cl2


Coordination complex


Oxidation Number (x)



x+3(-2)= -3




x+6(0) = 3




x+2(0)+(-1)+0 = +1




x+5(0)+(-1) = +2

x= 3


Q3. What is the coordination number of Platinum in given complex[Pt(NH3)2Cl(NH2CH3)]Cl

A. 5
B. 6
C. 4
D. 8

Solution : As the ligand NH3,Cl,NH2CH3 are monodentate, it means one ligand will able to donate from one atom. There are four monodentate ligands given. Hence, in total four donor atoms are present around the Platinum for donation. Hence the coordination number is 4.

The correct answer is (C).

Q4. Calculate the oxidation number of the metal in the coordination sphere 

(A) [Pt(NH3)2Cl(NO2]
(B) K3[Cr(C2O4)3]
(C) Hg[Co(SCN)4]
(D) [Co(en)2(Cl)2]Cl


Coordination complex


Oxidation Number (x)







x+3(-2)= -3




x+4(-1)= -2




x+2(0)+2(-1) = +1


Frequently Asked Questions

Q1. What is the nature of the central atom?
Answer: The centre atoms or ions in coordination compounds are usually Lewis Acids, which can operate as electron-pair acceptors

Q2. What makes the coordination entity different from the coordination sphere?
Answer: The coordination entity and coordination sphere are parts of a coordination compound that contain the core atom and the ligands that surround it. The main distinction is the electrical charge of a coordination compound is not specified in the coordination entity but is specified in the coordination sphere.

Q3. What exactly is a Polynuclear complex?
Answer: Polynuclear complexes are coordination compounds with two or more metal atoms or ions contained in a single coordination sphere. Direct metal-metal bonds, bridging ligands, or both may be used to hold the two atoms together.

Example [Re2Cl8]2-, [(NH3)5CoO2Co(NH3)5]Cl5etc.

Q4. Why do some elements form coordination compounds while others do not?
Answer: The donation of electron pairs from ligands to metal ions produces coordination compounds and electron pairs from ligands can now be accepted by metal ions with unoccupied p or d orbitals. Transition elements with partially full d orbitals can easily form coordination compounds as a result.

Related Topics

Oxidation number of elements in coordination compounds

Crystal field theory

EAN Rule

Bonding in coordination compounds

Organometallic Compounds

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