GENERAL KNOWLEDGE

BONDING IN COMPLEX COMPOUNDS

Introduction

In chemistry, a complex compound is a molecule consisting of a central metal ion or atom, typically a transition metal, and a surrounding array of other molecules or ions known as ligands. The interaction between the metal ion and the ligands is known as coordination bonding or complexation.

There are several types of bonding involved in complex compounds, including:

  • Covalent Bonding: This type of bonding occurs when the metal ion shares electrons with the ligands to form a stable bond. Covalent bonding is typically seen in complexes with ligands that are small and have lone pairs of electrons, such as carbon monoxide or ammonia.
  • Ionic Bonding: In some cases, the metal ion and ligands can form an ionic bond, where the metal ion donates electrons to the ligands. This typically occurs when the ligands are larger and have a negative charge, such as chloride or sulfate ions.
  • Coordinate Covalent Bonding: This type of bonding occurs when a lone pair of electrons on a ligand is donated to the metal ion to form a stable bond. This type of bonding is also known as a dative bond or a coordinate bond.

The strength of the bonding in complex compounds can vary depending on the type of bond and the nature of the metal ion and ligands involved. The bonding can also be influenced by factors such as the size and shape of the ligands, the charge on the metal ion, and the overall geometry of the complex molecule.

 

Ligands and Central Ions

1) A ligand is a molecule or ion that binds to a central metal ion or atom, typically by donating one or more pairs of electrons to form a coordinate covalent bond. The ligand can be a simple molecule like water or ammonia, or a complex organic molecule like a protein. Ligands can also be anions, such as chloride ions, or cations, such as ammonium ions.

2) A central ion, also known as a central metal ion, is an atom or ion that is coordinated to one or more ligands to form a complex. Central ions are typically transition metal ions, such as iron, copper, and nickel, but they can also be main group elements like boron and aluminum. The central ion is usually positively charged, and its charge is balanced by the negative charge of the ligands that coordinate to it. The coordination complex formed by a central ion and its ligands is known as a coordination compound.

 

Examples of ligands

Ligands can be classified as mono-, bi-, or polydentate based on the number of atoms or functional groups that can coordinate with the metal ion.

Here are some examples of ligands:

  1. Water (H2O) – Water is one of the most common ligands in chemistry, and it can coordinate with metal ions to form hydrated metal complexes.
  2. Ammonia (NH3) – Ammonia is a common ligand that can coordinate with metal ions to form metal-ammonia complexes. These complexes are often used in coordination chemistry and in the synthesis of catalysts.
  3. Carbon monoxide (CO) – Carbon monoxide is a poisonous gas, but it can also act as a ligand in coordination complexes. Metal-carbonyl complexes are important in catalysis and organic synthesis.
  4. Ethylenediamine (en) – Ethylenediamine is a bidentate ligand that can coordinate with metal ions through its two nitrogen atoms. It is commonly used in the synthesis of metal complexes and as a chelating agent.
  5. EDTA – Ethylenediaminetetraacetic acid (EDTA) is a polydentate ligand that can coordinate with metal ions through its four carboxylate groups and two amine groups. It is commonly used as a chelating agent to remove metal ions from solutions.
  6. Cyanide (CN-) – Cyanide is a toxic ligand, but it can also act as a strong pi-acceptor ligand in coordination complexes. Metal-cyanide complexes are important in bioinorganic chemistry and in the synthesis of materials.
  7. Phosphine (PH3) – Phosphine is a common ligand that can coordinate with metal ions to form metal-phosphine complexes. These complexes are often used in homogeneous catalysis and in the synthesis of organometallic compounds.

 

Formation of Coordination Compounds

Coordination compounds are formed when metal ions are surrounded by ligands, which are typically molecules or ions that donate a pair of electrons to the metal ion to form a coordinate bond. The formation of coordination compounds involves several steps, including:

  1. Formation of the metal ion: The metal ion must first be formed by dissociating from its original compound or by oxidizing a metal atom.
  2. Coordination of the ligands: The ligands coordinate to the metal ion by forming coordinate covalent bonds. Ligands that donate one pair of electrons to the metal ion are called monodentate ligands, while those that donate two or more pairs of electrons are called polydentate ligands.
  3. Formation of the coordination sphere: The metal ion and its ligands form a coordination sphere, which is a complex that includes the metal ion and its surrounding ligands.
  4. Formation of counter ions: The coordination sphere is typically neutral, so counter ions may be required to balance the charge of the complex.
  5. Naming the compound: The coordination compound is named by listing the ligands in alphabetical order, indicating the number of each ligand using Greek prefixes, and indicating the charge of the coordination sphere using Roman numerals.

 

Nomenclature of complex ions

Here are some guidelines for nomenclature of complex ions and compounds:

  1. Cations are named first, followed by anions.
  2. In a complex ion or compound, the central metal ion is named first, followed by the ligands in alphabetical order.
  3. Ligands are named by adding the suffix -o to the stem of the ligand name. For example, NH3 is named ammine, and H2O is named aqua.
  4. If there are multiple ligands of the same type, prefixes are used to indicate the number. For example, two Cl- ions are named dichloro, and three NO3- ions are named trinitro.
  5. If the ligand is negatively charged, the suffix -ate is added to the stem of the ligand name. For example, SO4 2- is named sulfate.
  6. If the ligand is neutral, the name is not modified.

 

Examples:

1) [Fe(H2O)6]2+ is named hexaaquairon(II) ion.

2) [CuCl4]2- is named tetrachlorocuprate(II) ion.

3) [Co(NH3)5Cl]2+ is named pentaamminechlorocobalt(III) ion.

4) [Cr(H2O)6]3+ is named hexaaquachromium(III) ion.

5) K2SO4 is named potassium sulfate.

6) Fe(NO3)3 is named iron(III) nitrate.