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Chapter 8 – Coordination Compounds

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Studies around coordination compounds have existed since the dawn of modern chemistry. Also called coordination complexes, these compounds play a crucial role in the chemical industry, especially in the bio-inorganic and inorganic areas.

Coordination compounds are chemicals that are made up of metal ions or atoms bound to several nonmetal molecules or anions. Here, the central metal atom is known as the coordination center, and the other molecules are referred to as complexing agents or ligands. Various transition metals (d- block elements) form coordination complexes. Examples of commonly known coordination compounds are hemoglobin, vitamin B12, and chlorophyll.

Werner’s Theory of Coordination Compounds

Alfred Werner, a Swiss chemist, proposed a theory regarding the structure of coordination complexes. He conducted experiments to study the chemical and physical characteristics of these compounds. While experimenting with compounds of cobalt(III) chloride with ammonia (CoCl3.xNH3), Werner found that some chloride ions precipitated as silver chloride on the addition of silver nitrate solution, but some remained in the solution only. Based on these observations, Werner’s theory of coordination compounds was formulated:

  • Coordination compounds exhibit two kinds of valances (linkages): primary & secondary.
  • While the primary valences are generally ionizable and are fulfilled by anions/negative ions, the secondary valences are non-ionizable and are fulfilled by negative ions or neutral molecules.
  • The secondary is constant for a metal and is equal to the coordination number of a compound.
  • The ions linked by secondary valence have specific spatial characteristics that depend upon the coordination number.

The nomenclature of coordination compounds follows from the rules given by IUPAC (International Union of Pure and Applied Chemistry). These naming formulas let you know about the constitution of a compound concisely. The basic rules for coordination complexes are listed below:

  • Coordination complexes are named with ligands listed alphabetically before the central metal ion.
  • To show the number of different types of ligands, prefixes like mono, di, tri, etc., are attached to its name. However, for complex (polydentate) ligands, prefixes like bis, tris, tetrakis, pentakis-, etc. are used.
  • The oxidation state of the central metal atom is written in Roman numerals in parenthesis.
  • In cases where anions and cations are present in the compound, the names of cations are stated before the names of anions.
  • For anionic complexes, the metal’s name is suffixed with ‘-ate’, but for cationic and neutral complexes, the metal is named after the element only. In some cases, Latin names of metals are used.

Example: [Ag(NH3)2][Ag(CN)2] is written as: diamminesilver(I) dicyanoargentate(I)

When two or more compounds have the same molecular formula but different atomic arrangements, they are termed as isomers. Isomers differ in a few chemical and physical properties due to the difference in their structures. Isomerism in coordination compounds prevails mainly in two ways: stereoisomerism and structural isomerism.

Stereoisomers have the same position of atoms/ions but vary in their spatial orientation around the central metal ion. These are further divided into geometrical isomers (several possible geometric alignments of ligands) or optical isomers (non-superimposable mirror images).

Structural isomers have the same chemical formula but a different set of bonds. It is further classified into solvate isomerism (varying number of the solvent molecules attached to the metal atom), ionization isomerism (interchange of ligand and counter ion), and coordination isomerism (interchange of ligands between different metal ions). Apart from these, various other types of isomerism exist among coordination complexes depending upon their chemical composition.

Coordination compounds are highly valued in metallurgical processes as they aid in extraction and purification of precious metals. Also, they are useful to the pigment and dying industry due to their coloured varieties. Lately, coordination compounds are being utilized in medicinal chemistry.

1.     What is the nature of bonding in coordination compounds?

Many theories shed light on the nature of bonding in coordination compounds. Some of the popular ones are Valence Bond Theory (VBT) and Crystal Field Theory (CFT).

  • According to VBT, the central metal atom or ion under the impact of ligands uses its partially filled orbitals to form equivalent orbitals of specific geometry like tetrahedral, octahedral, etc. These hybridised orbitals may overlap with ligand orbitals which can donate electron pairs for covalent bonding. The magnetic behavior of coordination complexes is predicted on the basis of VBT.
  • On the other hand, CFT is often used to explain the colour of coordination compounds. CFT elucidates that the degeneracy of d- or f- orbitals are broken by the electric field of the ligands, resulting in a stable complex. This phenomenon is called crystal field splitting. Without the ligands, the compound is colourless.

2.     What is the kind of bonding in metal carbonyls?

When carbon monoxide serves as a ligand, the coordination compounds are generally called metal carbonyls. These complexes have well-defined structures and synergic bonding between the metal and ligand. This synergic effect is created because of the π and σ character of the bond. As stated, the vacant orbital of the metal ion overlaps with filled orbitals of a carbon atom and vice-versa.

3.     How is the stability of coordination compounds determined?

The degree of association of the metal ion and ligands is determined by stability (or formation) constant. Coordination complexes are formed in a number of steps that are reversible in nature. Each step gives rise to an equilibrium constant depending upon the free metal ions and ligand molecules. Higher the stability constant, more is the stability of the complex.

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