1. Double Salt: When dissolved in water, a double salt is created by mixing two separate salts that crystallise as a single material yet ionise as two distinct salts. These salts lose their identity in solution, which means they test positive for all the ions present in the salt when dissolved in water. For example, Mohr's salt and potash alum.

 

Example: 

FeSO₄.(NH₄)2SO₄.6H₂O Mohr's salt → Fe^{2 +}_(aq) +  6H₂O  +  2NH₄ ⁺ _(aq) +  2SO₄^2--(aq)

 

A coordination compound is a molecular molecule that is made up of two or more simple molecular compounds and preserves its identity in both solid and dissolved states.

Example: 

[Cu(NH₃)₄]SO₄⇌[Cu(NH₃)₄]²⁺ +  SO₄²⁻

A coordination compound is made up of a ligand, a central atom, a complex ion, a cation, or an anion. The complex ion is usually written in a square box, while the ion (cation or anion) is usually written outside the complex ion.

Example:

[Co(NH₃)₆ ] Cl₃ [Co(NH₃)₆ ] Cl₃ 

[Complex ion] anion[Complex ion] anion  

General Formula: Ax[MLn]/[MLn]ByAx[MLn]/[MLn]By where M is the central metal atom/ion, L is the ligand, A is the cation and B is the anion.

1. Coordination Entity: It is the primary metal atom or ion that is permanently bound to a specified number of ions or molecules. In the coordination entity [Co(NH₃)₆]Cl₃, for example, six ammonia molecules are surrounded by three chloride ions.
2. Central Atom/Ion: The primary cation is surrounding and coordinately bound to one or more neutral molecules or negatively charged ions in a precise geometrical arrangement. Co³⁺ is the primary positively charged metal ion in the complex [Co(NH₃)₆]Cl₃, for example, and it is coordinately bonded to six neutral NH₃ molecules within the coordination sphere. Lewis acid refers to the core metal/ion.
3. Ligands: Ligands are ions or molecules that are bonded to the central atom/ion of the coordination entity. Simple ions, such as Cl, and small molecules, such as H₂O or NH₃, or bigger molecules like H₂NCH₂CH₂NH₂ are used.
4. Coordination Number (C.N): The coordination number refers to the number of atoms in the ligands that are directly attached by coordinate bonds to the core metal atom or ion. It is the same thing as secondary valency.
5. Coordination Sphere: The central metal atom or ion and the ligands that are directly attached to it are enclosed by a square bracket. It was named after the coordination sphere, or the first sphere of attraction. The metal ion behaves as a single unit because it tightly binds in the coordination sphere's ligands.
6. Coordination Polyhedron: The spatial arrangement of the ligand atoms that are directly connected to the centre atom/ion is known as a coordination polyhedron. For example, [Co(NH₃)₆]^{3 +} is octahedral, whereas [Ni(CO)₄] is tetrahedral and [PtCl₄]₂ is square planar.
7. Central Metal Atom Oxidation Number: It is the charge that would be left on the central metal ion if all ligands and electron pairs were removed. It is computed like this:

 

Example:

K₄[Fe(CN)₆]→4K⁺+[Fe(CN)₆]⁴−K₄[Fe(CN)₆]→4K⁺+[Fe(CN)₆]⁴⁻

The charge on the complex ion is -4.

Let the charge on Fe be x.

Now, the charge on cyanide ions is -1.

⇒x+6×(−1)=−4⇒x+6×(−1)=−4

⇒x=+2⇒x=+2 

Hence, the oxidation number of Fe is +2 (II).

    8. Homoleptic and Heteroleptic Complexes: Homoleptic complexes, such as [Co(NH₃)₆]³⁺,have a central atom that is                        coordinated with only one type of ligand. Heteroleptic complexes, such as [Co(NH₃)₄Cl₂]⁺, have the central atom coordinated          with more than one type of ligand.

Nomenclature 

The following rules are followed when naming a complex ion:

     A. The cations are named first, then the anions.

     B. The oxidation state (O.S.) of the central metal ion is indicated by a Roman numeral.

     C. The names of the ligands are listed first, then the name of the central metal ion.

     D. Anion ligand names that end in 'ide' are replaced with 'o', 'ite' is replaced with 'ito’, and 'ate' is replaced with 'ato’.

     E. For many ligands that are molecules, the unaltered term is used.

     F. –ium is used to end positive groupings. NH2NH3+ hydrazinium, for example.

     G. The prefixes di, tri, tetra, penta, and hexa are used to indicate the number of ligands of a certain type when there are                        multiples of that kind. There is an exception when the ligand's name contains a number, such as in ethylenediamine. Bis, tris,            and tetrakis are used instead of di, tri, and tetra, and the ligand name is put in brackets to avoid confusion.

     H. In the case of a complex anion, the metal is followed by 'ate'. 

          [Ni(CN)₄]²⁻: tetracyanonickelate (II) ion 

          Lead – plumbate 

          Gold – aurate 

          Zinc – zincate 

          Tin – stannate 

          Silver – argentate 

          Cobalt – cobaltate 

          Aluminium – aluminate 

          Iron – ferrate 

          Copper – cuprate 

          Manganese – manganate 

          Platinum – platinate

          Chromium – chromate

      I. If a complex consists of numerous metal atoms, it is said to be polynuclear. The cross-over ligands that join the two metal               atoms are denoted by the prefix.

      J. Different atoms can be used to join ambidentate ligands.–

          M←NO₂

          M←ONO

      K. When writing (not naming) the complex formula, use the following format:

• • Square brackets should be used to surround the complex ion.
  • Ligands are arranged alphabetically after metals, with negative ligands appearing first, followed by neutral and positive ligands.

Werner understood the meaning of bonding in complexes and concluded that the metal in complexes has two valencies.

       1. ƒˇPrimary Valency: The primary valency is determined by the oxidation state of the central metal atom or ion. These are                asymmetrical in nature.

          Example: What are the primary valencies of K₄[Fe(CN)₆] and [Cu(NH₃)₄]SO₄K₄[Fe(CN)₆] and [Cu(NH₃)₄]SO₄?

          Sol. The primary valency of K₄[Fe(CN)₆] and [Cu(NH₃)₄]SO₄K₄[Fe(CN)₆] and [Cu(NH₃)₄]SO₄ is 2.

      2. Secondary Valency: The number of ligand atoms coordinated to the central metal atom is referred to as secondary valency.            A complex ion has a specific form because it is directional.

         Example: What are the secondary valencies of [Co(NH₃)₆]Cl₃ and K₄[Fe(CN)₆][Co(NH₃)₆]Cl₃ and K₄[Fe(CN)₆]? 

         Sol. The secondary valency of [Co(NH₃)₆]Cl₃[Co(NH₃)₆]Cl₃ is 6.

 

Six ligands are coordinated to Fe in K₄[Fe(CN)₆]. As a result, the secondary valency is 6. The primary valency is satisfied by ions attached to complex ions. Dotted lines are used to represent it. Primary valency is also known as ionisable valency. The ligands are non-ionisable and are represented by a solid line because they satisfy the secondary valency. 

Every element has both primary and secondary valencies that it can satisfy. The coordination sphere exhibits dual behaviour when a negative ion is present. It can satisfy primary as well as secondary valencies.

The secondary valencies are fulfilled by ligands that are focused at specific regions in space. The coordination number determines the geometry of the complex ion. The complex is octahedral if the metal has coordination number 6, which indicates that the ligands' six donor atoms occupy six places around the metal octahedrally. If the coordination number is 4, the geometry of the complex can be tetrahedral or square planar. Different types of isomerism would exist in coordination compounds, according to this theory.