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Coordination compounds are a class of substances with a unique chemical structure that features a central metal atom enclosed by non-metal atoms or groups of atoms called ligands. Chemical bonding is seen between the ligands and the central metal atom. Coordination compounds comprise substances like haemoglobin, vitamin B12, dyes and pigments, chlorophyll, and catalysts used in manufacturing organic substances.
In other words, the compounds that contain a coordination complex are called coordination compounds. A coordination complex is the product of a Lewis acid-base reaction in which neutral molecules or anions referred as ligands are bonded to a central metal atom or ion by coordinate covalent bonds.
Coordination number is the number that represents the number of donor atoms that are bonded to the central metal atom/ion.
How To Calculate Coordination Number
It is an easy task to find the coordination number of the compound if the concept is applied right.
Example 1: To find the coordination number of [Zn(CN)4]2-
First, classify the Lewis base and Lewis acid.
As per the formula, [Zn(CN)4]2- contains CN– lewis base and Zn2+ lewis acid. C is the donor atom.
Hence analyzing the chemical bonding between the donor atom and the central metal ion, the coordination number is found to be 4.
Example 2: To find the coordination number of [Ni(NH3)6]2+
First, classify the Lewis base and Lewis acid.
As per the formula, [Ni(NH3)6]2+ contains NH3 lewis base and Ni2+ lewis acid. N is the donor atom.
Hence, analyzing the donor atom bonded to the central metal ion, the coordination number is found to be 6.
Naming Coordination Compounds
While naming the coordination compound remember these three basic rules:
- Name the Ligands first according to the alphabetical order.
- Name of the metal after ligands.
- Followed by the oxidation state of the metal, noted in parentheses (II, IV).
Here are some tables that help to name the coordination compound.
Table 1: Names of Anionic Ligands
| Anionic Ligands | Names |
| Br– | Bromo |
| F– | Fluoro |
| O2- | Oxo |
| OH– | Hydroxo |
| CN– | Cyano |
| C2O42- | Oxalato |
| CO32- | Carbonato |
| CH3COO– | Acetato |
Table 2: Names of Neutral Ligands
| Neutral Ligands | Names |
| NH3 | Ammine |
| H2O | Aqua |
| NO | Nitrosyl |
| CO | Carbonyl |
| O2 | Dioxygen |
| N2 | Dinitrogen |
| C5H5N | Pyridine |
| H2NCH2CH2NH2 | Ethylenediamine |
Table 3: Numerical Prefixes
| Number | Prefixes |
| 1 | mono |
| 2 | di (bis) |
| 3 | tri (tris) |
| 4 | tetra |
| 5 | penta |
| 6 | hexa |
| 7 | hepta |
| 8 | octa |
| 9 | nona |
| 10 | deca |
| 11 | undeca |
| 12 | dodeca |
Table 4: Name of metal
| Name of Metal | Name in an Anionic Complex |
| Iron | Ferrate |
| Lead | Plumbate |
| Tin | Stannate |
| Copper | Cuprate |
| Gold | Aurate |
| Silver | Argenate |
Let us look into some examples
Example 1:
Coordination compound: K4[Fe(CN)6]
Nomenclature : potassium hexacyanoferrate(II)
Solution: Here, potassium is the cation and the complex ion is the anion.
Since there are 4 K+ ions are associated with a complex ion, the charge on the complex ion is said to be 4.
Since each ligand carries –1 charge, the oxidation number of Fe must be +2.
Example 2:
Coordination compound: [Pt(NH3)5Cl]Br3
Nomenclature : pentaamminechloroplatinum(IV) bromide
Solution: Here, the complex ion is a cation, the anion is 3 bromides ions.
Hence, the charge of the complex ion is +3.
Since NH3 are neutral, – 1 charge carried by the chloride is considered. Hence, the oxidation number of platinum is +4.
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