Chemistry Of Coordination Compounds

1. Introduction

d block elements or Transition elements

These are the elements in which “(n-1) d orbital” is under the process of completion. For example, elements 5^th period 4d is under process of completion. Similarly, for elements of 6th period 5d is under process of completion.

Types of d-block elements

d-block elements have two types:

i) Non-typical: these are the elements whose d-block is completely filled. Zn, Cd, Hg are non-typical.

ii) Typical: all other elements of d-block except the non-typical ones are the typical elements having incomplete d-orbital. They have deficiency of electron. To accommodate maximum e^-s in d-orbital species can receive electron pair from other species. This acceptance of e pair results in the formation of coordination compounds.

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Ligand:

Any atom or group of atoms having electron pair to donate to the central metal atom or ion is called ligand or electron pair donor.

L: → M+ (ions) [coordination complex / compound]

Chemistry of coordination compounds/ Introduction

Figure 1.1: General representation of a metal-ligand complex

The complex is written in square bracket called coordination sphere. It has charge which is equal to the sum of all the charges of atom within the sphere. It is also known as net charge (of ligand & metal atom).

Properties of Ligands

Neutral, anionic or cationic
have no relation with charge
Presence of lone pair is necessary for ligand
Example: Neutral ligand: H₂O, NH₃, CO. Anionic ligand: Cl^-, OH^-. cationic ligand: H₃N⁺-NH₂
They can be monodentate: donate 1 electron pair, e.g. NH₃or water [ water has 2 electron pairs but it can donate only one so it’s a monodentate].
They can be bidentate: having 2 lone pair and so on.

Figure 1.2: A bidentate

coordination number have no relation with ligand.

Coordination number:

The number of coordinate covalent bonds formed by metal is called coordination number or It is the number of sites from where a metal can receive lone pair. For example, the molecule below has coordination number= 6.

Chelatingagents:

All the ligands having more than one donor sites are called chelating agents. So, all chelating agents are ligands but all ligands are not chelating agents. Complexes made by chelating agents. are called chelates. Chelating agent forms complexes mostly in ring form.