Coordination Compounds and It’s Terms

It is important to know about and thoroughly understand the important terms related to coordination compounds in chemistry. A coordination entity is defined as an arrangement in which a central atom or ion is bound to a fixed number of molecules or ions. The central cobalt ion, for example, is coupled to three ammonia molecules and three chloride ions in the coordination entity [CoCl3(NH3)3], [PtCl2(NH3)2], [Fe(CN)6]4–, [Ni(CO)4] and [Co(NH3)6]3+.

What are the important terms related to coordination compounds?

1. Coordination entity: A coordination entity is a central  metal atom or ion with a fixed number of ions or molecules linked to it. [CoCl3 (NH3)3], for example, is a coordination entity containing three ammonia molecules and three chloride ions around the cobalt ion. [Ni(CO)4], [PtCl2 (NH3)2], [Fe(CN)6]4–, and [Co(NH3)6]3+ are some more examples.

2. Central atom/ion: In a coordination entity, the central atom or ion is the atom or ion to which a fixed number of ions/groups are linked in a definite geometrical configuration around it. The central atoms/ions in the coordination entities [NiCl2(H2O)4], [CoCl(NH3)5]2+, and [Fe(CN)6]3–, respectively, are Ni2+, Co3+, and Fe3+. These central atoms/ions are known as Lewis acids.

3. Ligands: Ligands are ions or molecules attached to the central atom or ion of the coordination entity. Simple ions such as Cl–, small molecules such as H2O or NH3, larger molecules such as H2 NCH2CH2NH2 or N(CH2 CH2 NH2)3, and even macromolecules such as proteins are examples of ligands. When a single donor atom, such as Cl–, H2O, or NH3, binds the ligand to a metal ion, it is considered unidentate. When a ligand attaches two donor atoms, such as H2NCH2 CH2NH2 (ethane-1,2-diamine) or C2O42– (oxalate), or when many donor atoms are present in a single ligand, such as N(CH2 CH2 NH2)3, the ligand is said to be didentate.

For example: The hexadentate ligand ethylenediaminetetraacetate ion (EDTA4–) is a significant hexadentate ligand. It can bind two nitrogen and four oxygen atoms to a central metal ion. Chelate ligands are di- or polydentate ligands that bind a single metal ion with two or more donor atoms at the same time.

The denticity of the ligand refers to the number of such ligating groups. Chelate complexes, as opposed to comparable complexes having unidentified ligands, are more stable. Ambidentate ligands have two distinct donor atoms and can bind to any of the two ligands in the complex. The ions NO2– and SCN– are examples of such ligands. NO2 ions can bind with nitrogen or oxygen to form a molecule.

• Similarly, the sulphur or the nitrogen atom can coordinate the SCN– ion.

4. Coordination number: A metal ion’s coordination number is the number of ligand donor atoms to which it is directly bonded (CN). In their complex ions [PtCl6]2– and [Ni(NH3)4]2+, Pt and Ni have 6 and 4 coordination numbers, respectively. Both Fe and Co have a coordination number of 6 in the complex ions [Fe(C2O4)3]3– and [Co(en)3]3+, because C2O4 2– and en (ethane-1,2-diamine) are bidentate ligands.

5. Coordination sphere: The coordination sphere is made up of the centre atom or ion and the ligands attached to it, which are all surrounded in square brackets. The ionisable groups are referred to as counter ions because they are written outside the bracket. The coordination sphere in the complex K4 [Fe(CN)6] is [Fe(CN)6]4–, and the counter ion is K+.

6. Coordination polyhedron:  A coordination polyhedron is defined as a pattern of  arrangement of ligand atoms that are directly linked to the central atom/ion. Common examples of coordination polyhedrons are octahedral, square planar, and tetrahedral. [Co(NH3)6]3+, [Ni(CO)4], and [PtCl4]2–, for example, are octahedral, tetrahedral, and square planar, respectively. 

7. Oxidation number of the central atom: The central atom in a complex is the charge it carries when the ligands and electron pairs that it shares are eliminated. Following the name of the coordinating entity, a Roman numeral in parenthesis represents the oxidation number. Copper’s oxidation number in [Cu(CN)4]3–, for example, is +1, and it is written as Cu (I)

8. Homoleptic and heteroleptic complexes: Homoleptic complexes are those in which a metal is attached to only one type of donor group, such as [Co(NH3)6]3+. Heteroleptic complexes are those in which a metal is attached to more than one type of donor group, such as [Co(NH3)4 Cl2]+.

Conclusion

Coordination complex is a type of substance in which a central metal atom is surrounded by nonmetal atoms or groups of atoms called ligands that are chemically linked to it. Coordination compounds have a few key terms. Ligands, Coordination Number, Coordination Sphere, Coordination Polyhedron, Oxidation Number, Homoleptic and Heteroleptic Complex, Coordination Number, Coordination Sphere, Coordination Polyhedron. A coordination complex consists of a central atom or ion known as the coordination centre, which is usually metallic, and bound to molecules or ions known as ligands or complexing agents. Coordination complexes are metal-containing compounds that contain transition metals (elements in the Periodic Table’s d-block, such as titanium).