# What are Coordination Compounds ?

Coordination compound contains a central metal ion surrounded by number of oppositely charged ions or neutral molecules. E.g. [Cu(NH3)4]2+ In this complex copper is the central metal ion while ammonia is a neutral molecule.

## Definition Of Some Important Terms Pertaining To Coordination Compounds

• Coordination entity : A coordination compound constitutes a central metal atom or ion bonded to a fixed number of ions or molecules, For example : [Fe(CN)6]4.
• Central atom/ion : In a coordination entity, the atom or to which a fixed number of ion/groups are bound in a definite geometrical arrangement around it, is called the central atom/ion.
• Coordination numbers : The coordination numbers (CN) of a metal ion in a complex can be defined as number of ligand donor atoms to which the metal is directly bonded.
• Coordination Sphere : The central atom or ion and the ligands attached to it are enclosed in square brackets and is collectively termed as the coordination sphere. The ionisable groups are written outside the bracket and are called counter ion.

## Iupac Nomenclature Of Mononuclear Coordination Compounds

• The Central atom is listed first.
• The ligands are then listed in alphabetical order. The placement of ligands in the list does not depends on its charge.
• Polydentate ligands are also listed alphabetically.
• The formula for the entire coordination entity is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligands abbreviation are also enclosed in parentheses.
• There should be no space between ligands and the metal within a coordination sphere.
• When the formula of a charged coordination entity is to be written without counter ion, the charge in indicated outside the square brackets as a right superscript with the number before the sign.
• The charge of cation(s) is balanced by the charge of anion(s).

## Coordination Number

It is the total number of ligands attached to the central metal atom through coordinate bonds or the number of atoms of a ligand attached to the same central atom, e.g. hexadentate ligand should be counted as forming six coordination bonds.

 Charge of central atom Coordination number Ag +1 2 Cu +2 4, 6 Bi +3 6, 4 Zr +4 8, 6

## Coordination Sphere

Central atom and ligands comprise the inner coordination sphere; complex ion enclosed in square bracked, it behaves as a single unit.

## Ionization Sphere

Part of compound present outside coordination sphere, an outer coordination sphere constitutes a positive or negatively charged ions that are on more distance from the central ion or associated there with :

## Type Of Ligands

On the basis of bonding sites or number of donor atoms. present, ligands can be classified in following categories :

• Unidentate ligand : When a ligand is bond to a metal ion through a single donor atom, as with Cl–, H2O or NH3, the ligand is said to be unidentate.
• Didentate ligands : H2NCH2CH2NH2 (ethane-1, 2-diamine) or C2O4 2– (oxalate),
• Polydentate : Ethylenediaminetetraacetate ion (EDTA4–) is an important hexadentate ligand.

Shapes of C oordination Compounds

 Coordination Number Shape Example 2 Linear [CuCl 2], [Ag(NH 2) 2]+, [AuCl 2]. 4 Square planar [Ni(CN) 4] 2–, [PdCl 4] 2–, [Pt(NH3) 4] 2+, [Cu(NH3) 4] 2+ 4 etrahedral [Cu(CN) 4] 3–, [Zn(NH3) 4] 2+, [CdCl 4] 2–, [MnCl 4] 2– 6 Octahedral [Ti(H2O) 6] 3+, (V(CN) 6] 4–, [Cr(NH 3) 4Cl 2] +, [Mn(H 2O 6] 2+, [FeCl 6] 3–, [Co(en) 3] 3+

## Crystal Field Theory

In the crystal field theory, the following assumption are made :

• Ligands are treated as point charges.
• There is no interaction between metals orbitals and ligand orbitals.
• The arrangement of the ligands around the central metal ion is such that the repulsion between these negative points is minimum.
• Splitting of d-orbital energies : The five d-orbitals in an isolated gaseous metal atom/ion are degenerate i.e. they have equal energy. If a spherically symmetric field of negative charges is placed around the metal, the orbitals will remain degenerate, but all of them will be raised in energy as a result of repulsion between the negative field and the electrons in the orbitals.

## Valence Bond Theory

This theory assumes that :

• The overlapping of two half-filled valence orbitals of two different atoms results in the formation of the covalent bond. The overlapping causes the electron density between two bonded atoms to increase. This gives the property of stability to the molecule, greater the extent of overlapping, stronger is the bond formed.
• In case the atomic orbitals possess more than one unpaired electron, more than one bond can be formed and electrons paired in the valence shell cannot take part in such a bond formation.
• The direction of the covalent bond is along the region of overlapping of the atomic orbitals, i.e. a covalent bond is directional.
• Based on the pattern of overlapping, there are two types of covalant bonds : sigma bond (σ-bond) and a pi bond (TT-bond).

## Werner’s Theory

• In coordination compounds, the metal ions exhibits and satisfied by two types of valency for bonding (i) Primary valency (oxidation state of metal ion)- positive charge of the metal ion is balanced by negative ions in the compound. It is ionizable and non-directional. (ii) Secondary valency (coordination number of metal ion)- molecules or ion (ligands) are attached directly to the metal ion. It is non-ionizable and directional.
• Every metal has a fixed number of secondary valency i.e. it has a fixed coordination number.
• Secondary valency directed towards fixed positions to give definite geometry of the coordination compounds.

## Colour Of Coordination Compounds

The colour of complexes is due to absorption of light in visible region of sepctrum and radiation of complementary colour. The energy is absorbed by electrons present in d-orbitals and they get excited to higher energy d-orbitals from lower energy d-orbitals. They radiate energy, when they come back to lower energy d-orbitals.

## Magnetic Properties Of Coordination Compounds

The extent of paramagnetism is measured in terms of the magnetic moment, μ. The larger the magnitude of μ, greater is the paramagnetism of the compound. Magnetic moment has contributions from spin and orbital angular momentum. A non-spherical environment may lead to quenching of the contribution from orbital
angular momentum. However, the spin-only magnetic moment survives in all cases and is related to the total number of unpaired electrons.

$$\mu_{\text{eff}}= \mu_{s,o}= 2\sqrt{\text{S}(\text{S+1})}= \sqrt{\text{n}(\text{n+2})}\text{BM}$$