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.