Coordination number

The coordination number in coordination compounds refers to the total number of ligand atoms that are directly bonded to the central metal atom or ion. Understanding the coordination number is essential for predicting the geometry, stability, and reactivity of coordination compounds. Let's explore this concept in more detail:

1.0Definition of Coordination Number

The coordination number of a metal ion in a coordination compound is the number of ligand donor atoms to which the metal is directly bonded. This number is crucial in determining the shape or geometry of the coordination complex.

2.0Common Coordination Numbers and Geometries

The coordination number of a central metal ion in a complex typically ranges from 2 to 8. Each coordination number is associated with a specific geometric arrangement of the ligands around the central metal ion:

Here are some compounds with their coordination number and Geometry:

3.0Factors Affecting Coordination Number

Several factors influence the coordination number of a central metal atom/ion in a coordination complex:

• Size of the Central Metal Ion: Larger metal ions can accommodate more ligands around them, leading to a higher coordination number.
• Charge on the Metal Ion: A higher positive charge on the metal ion typically increases its ability to attract more ligands.
• Size and Electron Donating Ability of Ligands: Bulky ligands may lead to lower coordination numbers due to steric hindrance. Similarly, stronger ligands that form more stable bonds can support higher coordination numbers.
• Electronic Configuration of the Metal Ion: The availability of empty orbitals in the metal ion for bonding with ligands also determines the coordination number.

4.0Importance of Coordination Number

1. Determines Geometry

The coordination number directly influences the spatial arrangement of ligands around the metal centre, dictating the geometry of the complex.

Example: [Cu(NH₃)₄]²⁺

This complex has a coordination number of 4. Due to the d⁹ configuration of Cu²⁺, it adopts a square planar geometry. Square planar geometries are common for coordination number 4 when dealing with d⁸ metals like Pt²⁺ and Pd²⁺ as well.

2. Affects Physical and Chemical Properties

The coordination number and resulting geometry can affect the magnetic, optical, and reactivity properties of the coordination compounds.

Magnetic Properties:

• Example: [NiCl₄]^2- (tetrahedral) vs. [Ni(CN)₄]^2ー (square planar)[NiCl₄]^2- has a coordination number of 4 with a tetrahedral geometry and exhibits paramagnetic properties due to the presence of unpaired electrons. On the other hand, [Ni(CN)₄]^2-, with the same coordination number of 4 but a square planar geometry, is diamagnetic because the ligand field created by cyanide ions causes pairing of all electrons in the d-orbitals.

Optical Properties:

• Example: [Cr(H₂O)₆]³⁺ (octahedral) vs. [CrCl₄]^ー (tetrahedral)The octahedral complex [Cr(H₂O)₆]³⁺ absorbs light differently than the tetrahedral complex [CrCl₄]^ー, leading to different colors observed for these solutions. This difference in optical properties is due to the different ligand field strengths and splitting of d-orbitals in different geometries.

3. Stability and Reactivity 

Coordination number also plays a role in the stability of complexes. For instance, an octahedral complex with six ligands might be more stable than a square planar complex with four ligands, depending on the ligands and the central metal.

Example: [Co(NH₃)₆]³⁺ vs. [PtCl₄]^2ー

The complex [Co(NH₃)₆]³⁺ has a coordination number of 6 with an octahedral geometry, which is highly stable due to the strong ligand field created by ammonia, which is a neutral ligand. In contrast, [PtCl₄]^2-, which has a coordination number of 4 and a square planar geometry, is less stable because the chloride ions are weaker ligands and also because square planar complexes are generally more reactive due to their lower ligand density around the metal center.

5.0Solved  Examples of Coordination Number

Q. Determine the coordination number of the metal ion in the complex [Fe(CN)₆]⁴⁻.

Solution:

• Central Metal Ion: Fe (Iron)
• Ligands: CN (cyanide) ligands
• Number of Ligands: 6

Since there are six cyanide ligands bonded to the iron ion, the coordination number is 6.

Q. What is the coordination number of copper in the complex [Cu(NH₃)₄]²⁺?

Solution:

• Central Metal Ion: Cu (Copper)
• Ligands: NH₃ (ammonia) ligands
• Number of Ligands: 4

Copper is bonded to four ammonia ligands, so the coordination number is 4.

6.0Coordination Numbers in Biological Systems

In biological systems, coordination numbers play a significant role in the structure and function of metalloproteins and enzymes. For example:

• Hemoglobin has an iron center with a coordination number of 6, four of which are bonded to nitrogen atoms in a porphyrin ring, one to a histidine residue, and the sixth site is available for oxygen binding.
• Vitamin B12 contains a cobalt ion coordinated to a planar corrin ring with a coordination number of 6.

7.0Special Cases and Exceptions of Coordination Number

• Pseudo-coordination Numbers: Sometimes, the coordination number is not an integer due to delocalized or weak bonding interactions.
• Polynuclear Complexes: In complexes with multiple metal centres, the coordination number might refer to the number of bonds formed between the metals and bridging ligands.