Overview of Coordination Compounds

A molecular or addition compound in which a central metal atom or ion is permanently attached to ligands through coordinate bonds is called a coordination compound.

An atom, ion or molecule which donates at least one lone pair of electrons to the central metal atom or ion and gets attached to it through a coordinate bond is called a ligand.

An electrically charged species formed by the union of a central metal atom or ion with one or more ligands is called a complex ion.

The part of a coordination compound consisting of the central metal atom or ion and the ligands directly attached to it, enclosed in square brackets, is called the coordination sphere.

The spatial arrangement of ligands directly attached to the central metal atom or ion is called the coordination polyhedron.

A complex ion carrying a net negative charge is called an anionic complex ion.

A coordination compound or complex which carries no net charge and does not ionise in solution is called a neutral complex.

The maximum number of ligands that can be directly coordinated to a central metal atom or ion is called the coordination number.

The net charge carried by a complex ion, equal to the algebraic sum of charges on the central metal ion and ligands, is called the charge number of the complex ion.

The electrical charge which the central metal atom actually has or appears to have in a coordination compound is called the oxidation number or oxidation state of the central metal atom.

A coordination compound in which the central metal atom or ion is bonded to only one kind of ligand is called a homoleptic complex.

A coordination compound in which the central metal atom or ion is bonded to more than one kind of ligand is called a heteroleptic complex.

When a polydentate ligand attaches itself to a central metal ion through two or more donor atoms in such a way that it forms a five- or six-membered ring with the central metal ion, the ligand is called a chelating ligand.

The coordination compound formed when a chelating ligand binds to a central metal atom or ion forming one or more five- or six-membered rings is called a chelate.

Werner’s theory of coordination compounds is a theory proposed by Alfred Werner (1892) to explain the nature, structure and properties of coordination compounds on the basis of primary and secondary valencies of the central metal atom or ion.

• Two types of valencies
  The central metal atom or ion present in a coordination compound exhibits two types of valencies, namely primary valency and secondary valency.
• Primary valency
  Primary valency is ionisable in nature, corresponds to the oxidation state of the central metal atom or ion, and is always satisfied by negative ions which ionise in solution.
• Secondary valency
  Secondary valency is non-ionisable, corresponds to the coordination number of the central metal atom or ion, and is satisfied by negative ions or neutral molecules, which do not ionise in solution.
• Directional nature
  The secondary valencies are directional and point in definite directions in space, whereas the primary valencies are non-directional.

1. Order of naming ions:

In ionic complexes, the positive ion (cation) is named first, followed by the negative ion (anion). The name of the complex part is written as a single word starting with a small letter.

2. Naming of ligands:

• Ligands are named before the central metal atom or ion.
• Neutral ligands are named as molecules (e.g. carbonyl, nitrosyl).
• Water is named aqua and ammonia is named ammine.

3. Negative ligands

Negative ligands are named by adding the suffix –o to the group name (e.g. chloro → chlorido, CN⁻ → cyanido, SO₄²⁻ → sulphato).

4. Number of ligands

When two or more ligands of the same type are present, prefixes di, tri, tetra, penta, hexa, etc., are used.
For polydentate ligands, prefixes bis, tris, tetrakis, etc., are used.

5. Order of naming ligands

When more than one type of ligand is present, ligands are named in alphabetical order, irrespective of their charge. Prefixes like di, tri, etc., are not considered while arranging alphabetically.

6. Naming of central metal atom or ion

• In cationic or neutral complexes, the metal is named by its common name, followed by its oxidation state in Roman numerals in brackets.
• In anionic complexes, the metal name ends with the suffix –ate, followed by its oxidation state in Roman numerals.

7. Special cases (ambident and bridging ligands)

• Ambident ligands are named according to the donor atom through which they are attached (e.g. nitro, nitrito).
• Bridging ligands are indicated by adding the prefix μ (mu) before the ligand name.

The phenomenon in which two or more substances have the same molecular formula but possess different chemical structures or different spatial arrangements of atoms or groups is called isomerism, and such substances are called isomers.

Structural isomerism is the type of isomerism in which two or more coordination compounds have the same molecular formula but differ in their structural arrangements.

When coordination compounds having the same molecular formula show a difference in their ionisation behaviour and furnish different ions in solution, the phenomenon is called ionisation isomerism.

When water molecules act either as ligands or as water of crystallisation, and complexes differ in this respect, they exhibit hydrate isomerism.

This type of isomerism arises when ligands having two different donor atoms coordinate with the central metal atom through different donor atoms. Such ligands are called ambident ligands.

Coordination isomerism is shown by complexes in which both positive and negative parts are complex species and arises due to the interchange of ligands between the coordination spheres.

This type of isomerism arises when the ligand present in the complex itself exists in more than one isomeric form.

Stereoisomerism arises due to different spatial arrangements of atoms or groups around the central metal atom. The isomers thus obtained are called stereoisomers.

Geometrical isomerism arises due to different spatial arrangements of similar groups around the central metal atom, resulting in cis and trans forms.

When similar groups are arranged on the same side of the central metal atom, the isomer formed is called the cis isomer.

When similar groups are arranged on opposite sides of the central metal atom, the isomer formed is called the trans isomer.

In octahedral complexes, when three identical ligands occupy one triangular face, the isomer is called a facial (fac) isomer.

When three identical ligands occupy positions around the edges of the octahedron, the isomer is called a meridional (mer) isomer.

Optical isomerism is shown by coordination compounds which possess chirality, that is, they do not possess any element of symmetry.

A molecule is said to be chiral if it does not possess any element of symmetry and its mirror image is not superimposable on itself.

The two forms of an optically active compound which are mirror images of each other are called enantiomers.

1. Basic concept of Valence Bond Theory:
   According to Valence Bond Theory, the formation of a coordination compound involves a reaction between a Lewis base (ligand) and a Lewis acid (central metal atom or ion), resulting in the formation of a coordinate (dative) covalent bond.
2. Vacant orbitals and coordination number:
   The central metal atom or ion provides a number of vacant atomic orbitals equal to its coordination number to accommodate electron pairs donated by ligands. Each monodentate ligand donates one pair of electrons.
3. Hybridisation of orbitals:
   The vacant atomic orbitals of the central metal atom or ion undergo hybridisation. During hybridisation, orbitals mix and redistribute their energies to form equivalent hybrid orbitals.
4. Overlap and bond formation:
   When ligands approach the central metal atom or ion, the hybrid orbitals overlap with ligand orbitals containing lone pairs of electrons, forming strong metal–ligand coordinate covalent bonds.
5. Non-bonding electrons:
   The non-bonding electrons of the central metal atom or ion remain unaffected and do not participate in bond formation.
6. Magnetic behaviour:
   A complex is paramagnetic if it contains one or more unpaired electrons, whereas it is diamagnetic if all electrons are paired.
7. Inner and outer orbital complexes:

• Complexes involving inner d-orbitals (d²sp³ hybridisation) are called inner orbital or low-spin complexes.
• Complexes involving outer d-orbitals (sp³d² hybridisation) are called outer orbital or high-spin complexes.

1. Basic assumption:
   Crystal Field Theory assumes that the interaction between the central metal ion and ligands is purely electrostatic (ionic) in nature.
2. Splitting of d-orbitals:
   When ligands approach the central metal ion, the five degenerate d-orbitals split into two sets of different energies due to electrostatic repulsion. This is called crystal field splitting.
3. Dependence on geometry:
   The pattern and extent of d-orbital splitting depend on the geometry of the complex (octahedral, tetrahedral, etc.) because d-orbitals differ in their spatial orientation.
4. Crystal field splitting energy (Δ or 10Dq):
   The energy difference between the two sets of split d-orbitals is called crystal field splitting energy (Δ or 10Dq). Its magnitude depends on the nature of ligands and geometry of the complex.
5. Filling of electrons:
   Electrons occupy the split d-orbitals according to Hund’s rule of maximum multiplicity and the relative values of Δ and pairing energy (P).
6. High spin and low spin complexes:
   If Δ < P, electrons enter higher energy orbitals forming high spin complexes.
   If Δ > P, electrons pair up in lower energy orbitals forming low spin complexes.
7. Crystal Field Stabilisation Energy (CFSE):
   The stability of a complex can be explained quantitatively by crystal field stabilisation energy (CFSE), which depends on the distribution of electrons in the split d-orbitals.