In chemistry, the transitional elements/metals (primarily the d-block elements) give rise to a large number of complex compounds in which the metal atoms are bound to several anions (negatively charged ions) or neutral molecules. Such compounds, by the modern definition, are called coordination compounds. One of the basic terms used in Coordination Compound Chemistry is the coordination number. The coordination number (CN) of a metal ion/atom in a given complex may be defined as the number of ligand donor atoms to which the metal is bonded directly. Ligands are the ions (charged) or molecules (neutral) bound to the central atom/ion in the coordination compound.
As discussed earlier, the coordination number of a crystalline solid is the number of atoms, ions, or molecules that a central atom/ion holds in the crystalline solid or the coordination compound as its closest neighbours. For example, in the complex ions, [PtCl6]2– and [Fe(H2O)6]2+, the coordination number of Pt and Fe are 6 and 6 respectively.Here Pt and Fe have connected to six monodentate ligands Cl- and H2O respectively.
Taking another example, [Cr(NH3)2Cl2Br2]–. Here the central atom Cr has coordination number 6 again because the total number of atoms/ions/molecules bonded to Cr is found to be 6. In the complex ion [Co(en)3 ]3+, the coordination number Co is 6 because en (ethylenediamine) are bidentate ligands.
In a coordination compound, the ligands are attached to the central metal atom/ion through coordinate bonds. Therefore to calculate the coordination of the central metal atom/ion, we need to calculate the total number of coordinate bonds made by all the ligands with the metal atom/ion.
Let us take an example:
The compound [Pt(NH3)2Cl2]
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Here it is very clear that Pt carries two coordinate bonds with NH3 and two with Cl respectively. The total number of coordinate bonds made by all the ligands stands as 4. Hence, the coordination number (CN) of the compound [Pt(NH3)2Cl2] is 4.
The coordination number of the central metal atom/ion may be used to deduce the molecular geometry of the coordination compound. Based on the coordination number (CN), the molecular geometry of the coordination compounds are given as follows:
9 and above
Other complex structures like cuboctahedron
Some examples include:
[Ag(NH3)2]+ where Ag has coordination number 2 and the molecular geometry of the compound is linear.
[NiCl4]2− where Ni has coordination number 4 and the molecular geometry of the compound is square planar.
[CoCl6]3− where Co has coordination number 6 and the molecular geometry of the compound is octahedral.
[ZrF7]3− where Zr has coordination number 7 and the molecular geometry of the compound is pentagonal bipyramid.
[CoCl5]2− where Co has coordination number 5 and the molecular geometry of the compound is trigonal bipyramidal.
1. Write the name of the following complexes and give the coordination number of the central metal atom/ion.
Ans. A. The complex is named as pentaamminechlorocobalt(III) chloride and the coordination number is 6. The coordination sphere has a charge of 2+ and the NH3 ligand is neutral but the chloro ligand has a charge of 1−.
B. There are two ions of Na+ and so the coordination sphere of [PtCl6]2− has two negative charges. The coordination sphere consists of 6 anionic chloride ligands and the oxidation state of the platinum is 4+. The name of the complex is sodium hexachloroplatinate(IV), and the coordination number is 6.
C. The name is potassium trioxalatoferrate(III). The complex has a coordination number of 6. The oxalate ligands each have a charge of 2− and the coordination sphere has a charge of 3−.
2. What can you say about the molecular geometry of [Cu(CN)3]2−, [Ag(NH3)2]+ and [CoCl6]3− based on their coordination numbers?
Ans. In the molecular geometry of a coordination compound, we may use the coordination number.
In [Cu(CN)3]2−, the central metal atom Cu is bound by three coordinate bonds of CN. The coordination number of the compound is 3. Hence, the molecular geometry of [Cu(CN)3]2− is trigonal planar.
In[Ag(NH3)2]+, the central metal atom Ag is bound by two coordinate bonds of NH3. The coordination number of the compound is 2. Hence, the molecular geometry of [Ag(NH3)2]+ is linear.
In [CoCl6]3−, the central metal atom Co is bound by six-coordinate bonds of Cl. The coordination number of the compound is 6. Hence, the molecular geometry of [CoCl6]3− is octahedral.
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