1. Coordination Entity/Coordination Sphere :

Chapter 10:

coordination compounds

Addition Compounds :

They are formed by the combination of two or more stable compounds in stoichiometric ratio.

Double salts :

Those addition compounds which lose their identity in solution are called double salts.

K₂SO₄+ Al₂(SO₄)₃ + 24H₂O ®  K₂SO₄ . Al₂(SO₄)₃ . 24H₂O  2K⁺ (aq.) + 2Al⁺³ (aq.) + 4SO₄^2– (aq.)

Other examples are carnallite (KCl. MgCl₂ . 6H₂O), Mohr’s salt  [FeSO₄ . (NH₄)₂SO₄ . 6H₂O],

Potash alum [KAl(SO₄)₂.12H₂O] etc.

Coordination Compounds :

Those addition compounds which retain their identity (i.e. doesn’t lose their identity) in solution are called coordination compounds.

Fe(CN)₂ + 4KCN —® Fe(CN)₂ . 4KCN or K₄ [Fe(CN)₆] (aq.)  4K⁺ (aq.) + [Fe(CN)₆] ^4-(aq.)

Other examples are,  

[Cu(NH₃)₄]SO₄ (aq.)  [Cu(NH₃)₄]²⁺ (aq.) + SO₄^2– (aq.)

K₂[Zn(CN)₄] (aq.)  2K⁺ (aq.) + [Zn(CN)₄]^2– (aq.)

Coordination compound is defined as a species in which metal atom or ion is attached to group of neutral molecules / ions by coordinate covalent bonds.

Coordination Entity/Coordination Sphere :

A coordination entity constitutes a central atom/ion, usually of a metal, to which are attached a fixed number of other atoms or groups each of which is called a ligand. It may be neutral or charged. Examples being : [Co(NH₃)₆]³⁺, [PtCl₄]^2–, [Fe(CN)₆]^3–, [NiCl₂(OH₂)₄] and (NH₃), (Cl^–), (CN^–), (H₂O) are the ligands.

The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively called as coordination sphere.

Note : The remaining ions apart from complex ions i.e. outside the coordination sphere are called counter ions, free ions or ionisable ions.  For example, in K₄[Fe(CN)₆], the potassium (K⁺) ion is counter ion of coordination entity [Fe(CN)₆]^4–.

Central Atom/Ion :

In a coordination entity–the atom/ion to which are bound a fixed number of ligands in a definite geometrical arrangement around it, is called the central atom or ion. For example, the central atom/ion in the coordination entities : [NiCl₂(OH₂)₄], [CoCl(NH₃)₅]²⁺ and [Fe(CN)₆]^3– are Ni²⁺, Co³⁺ and Fe³⁺, respectively. These central atoms / ions are also referred to as Lewis acids.

Ligands :

The neutral molecules, anions or cations which are directly linked with central metal atom or ion in the coordination entity are called ligands.

These may be simple ions such as Br–, small molecules such as H₂O or NH₃, larger molecules such as H₂NCH₂CH₂NH₂ or N(CH₂CH₂NH₂)₃ or even macromolecules such as proteins.

When a ligand is attached to a metal atom ion through a single donor atom, as with Cl^–, H₂O or NH₃, the ligand is said to be unidentate. Similarly when a ligand is bound through two donor atoms, as in H₂NCH₂CH₂NH₂ (ethane-1, 2-diamine) or C₂O₄^2– (oxalate), the ligand is said to be bidentate and when several donor atoms are present in a single ligand as in N (CH₂CH₂NH₂)₃ or ethylenediaminetetraacetic acid (EDTA), the ligand is said to be polydentate.

Ambidentate Ligand :

Ligands which can ligate through two different atoms present in it are called ambidentate ligands. Examples of such ligands are the CN^–, NO₂^– and SCN^¯ ions. NO₂^– ion can coordinate through either the nitrogen or the oxygen atoms to a central metal atom/ion. Similarly, SCN¯ ion can coordinate through the sulphur or nitrogen atom. Such possibilities give rise to linkage isomerism in coordination compounds. For example,

                   nitrito-N

M ¬ O —N=O         nitrito-O

M¬ SCN                 thiocyanato or thiocyanato-S

M¬ NCS                isothiocyanato or thiocyanato-N

Chelate ligand :

Chelate ligand is a di or polydentate ligand which uses its two or more donor atoms to bind a single metal ion producing a ring. The complex formed is referred to as a chelate complex and the process of chelate formation is called chelation. The number of such ligating groups is called the denticity of  the ligand.