Chemistry Definitions of Some Important Terms Pertaining to Coordination Compounds
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Topics Covered :

● Coordination Entity
● Central Atom/Ion
● Ligands
● Coordination Number
● Coordination Sphere
● Coordination Polyhedron
● Oxidation Number
● Homoleptic and Heteroleptic

Coordination Entity :

`color{green}("Definition ")` : A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.

● `color{red}("Example ")` : (i) `color{red}([CoCl_3(NH_3)_3])` is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions.

(ii) Other examples are `color{red}([Ni(CO)_4], [PtCl_2(NH_3)_2], [Fe(CN)_6]^(4–), [Co(NH_3)_6]^(3+))`.

Central Atom/Ion :

`color{green}("Definition ")` : In a coordination entity, the atom/ion to which a fixed number of ions/groups are bound in a definite geometrical arrangement around it, is called the central atom or ion.

`color{red}("Example ")` : The central atom/ion in the coordination entities : `color{red}([NiCl_2(H_2O)_4], [CoCl(NH_3)_5]^(2+))` and `color{red}([Fe(CN)_6]^(3-))` are `color{red}(Ni^(2+), Co^(3+))` and `color{red}(Fe^(3+))`, respectively.

`color{red}("Note ")` : These central atoms/ions are also referred to as Lewis acids.

Ligands :

`color{green}("Definition" )` : The ions or molecules bound to the central atom/ion in the coordination entity are called ligands.

● These may be simple ions such as `color{red}(Cl^-)`, small molecules such as `color{red}(H_2O)` or `color{red}(NH_3)`, larger molecules such as `color{red}(H_2NCH_2CH_2NH_2)` or `color{red}(N(CH_2CH_2NH_2)_3)` or even macromolecules, such as proteins.

`color{green}("Types of Ligands based on Denticity ") : `

`color{green}("Denticity ")` : The number of ligating groups in a ligand which can bind to the central metal atom/ion is called the denticity of the ligand.

(i) `color{green}("Unidentate Ligands ")` : When a ligand is bound to a metal ion through a single donor atom, as with `color{red}(Cl^-)`, `color{red}(H_2O)` or `color{red}(NH_3)`, the ligand is said to be unidentate.

(ii) `color{green}("Didentate Ligands ")` : When a ligand can bind through two donor atoms as in `color{red}(H_2NCH_2CH_2NH_2)` (ethane-1,2-diamine) or `color{red}(C_2O_(4)^(2–))` (oxalate), the ligand is said to be didentate.

(iii) `color{green}("Polydentate Ligands ")` When several donor atoms are present in a single ligand as in `color{red}(N(CH_2CH_2NH_2)_3)`, the ligand is said
to be polydentate.

`color{red}("Example ")` : Ethylenediaminetetraacetate ion (`color{red}(EDTA^(4–))`) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

(iv) `color{green}("Chelate Ligands ")` : When a di- or polydentate ligand uses its two or more donor atoms to bind a single metal ion, it is said to be a chelate ligand.

● Complexes formed by chelating ligands are called chelate complexes.

● The complexes tend to be more stable than similar complexes containing unidentate ligands.

(v) `color{green}("Ambidentate Ligands ")` Ligand which can ligate through two different atoms is called ambidentate ligand.

● Examples of such ligands are the `color{red}(NO_(2)^-)` and `color{red}(SCN^-)` ions.

`->` `color{red}(NO_(2)^-)` ion can coordinate either through nitrogen or through oxygen to a central metal atom/ion.

`->` `color{red}(SCN^-)` ion can coordinate through the sulphur or nitrogen atom.

Coordination Number :

`color{green}("Definition ")` : The coordination number `color{red}("CN")` of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.

`color{red}("Example ")` (i) In the complex ions, `color{red}([PtCl_6]^(2-))` and `color{red}([Ni(NH_3)_4]^(2+))`, the coordination number of `Pt` and `color{red}(Ni)` are `6` and `4` respectively.

(ii) In the complex ions, `color{red}([Fe(C_2O_4)_3]^(3-))` and `color{red}([Co(en)_3]^(3+))`, the coordination number of both, `Fe` and `Co`, is `6` because `color{red}(C_2O_(4)^(2-))` and en (ethane-`1, 2`-diamine) are didentate ligands.

`color{red}("Note ")` : (i) Coordination number of the central atom/ion is determined only by the number of sigma bonds formed by the ligand with the central atom/ion.

(ii) `Pi` bonds, if formed between the ligand and the central atom/ion, are not counted for this purpose.

Coordination Sphere :

`color{green}("Definition ")` : The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively termed as the coordination sphere.

● The ionisable groups are written outside the bracket and are called counter ions.

● For example, in the complex `color{red}(K_4[Fe(CN)_6])`, the coordination sphere is `color{red}([Fe(CN)_6]^(4-))` and the counter ion is `color{red}(K^+)`.

Coordination Polyhedron :

`color{green}("Definition ")` : The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom.

● The most common coordination polyhedra are octahedral, square planar and tetrahedral.

● `color{red}("Example")` : `color{red}([Co(NH_3)_6]^(3+))` is octahedral, `color{red}([Ni(CO)_4])` is tetrahedral and `color{red}([PtCl_4]^(2-))` is square planar.

● Fig. 9.1 shows the shapes of different coordination polyhedra.

Oxidation Number of Central Atom :

`color{green}("Definition ")` : The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom.

● The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity.

● `color{red}("Example")` : Oxidation number of copper in `color{red}([Cu(CN)_4]^(3-))` is `+1` and it is written as `color{red}(Cu(I))`.

Homoleptic and Heteroleptic Complexes :

`color{green}("Homoleptic Complexes ")` : Complexes in which a metal is bound to only one kind of donor groups, e.g., `color{red}([Co(NH_3)_6]^(3+))`, are known as homoleptic.

`color{green}("Heteroleptic Complexes ")` : Complexes in which a metal is bound to more than one kind of donor groups, e.g., `color{red}([Co(NH_3)_4Cl_2]^+)`, are known as heteroleptic.